Semi-Synthetic oil


In a recent comment by Dude under the title of 'To flush or not to flush' there was useful logical advice on that topic.

I'm not so sure that his comment on semi-synthetic oil being a rip-off holds water. S-s oils are widely used by reputable service stations for oil changes and it was only when my YRV came out of warranty and I started servicing it myself that I changed to fully-synthetic. I can't say there has been any noticeable improvement, but I trust that the extra cost should bring some benefit.

My 'noticeable' observation on oil type relates to my Moto Morini 350cc twin motorcycle. It has a simple robust engine with no pressure feed to the rockers. Oil mist passes up the push rod tunnels, condenses on the inside of the rb cover and drips down from strategically placed 'crows feet' onto the rockers. Using the Castrol mineral oil traditionally used for this engine I found severe rocker face wear. I changed to fully-synthetic and the wear was almost totally banished. Out of interest I then changed to s-s and the wear rate remained the same as for f-s oil. As there was no wear benefit by using f-s instead of s-s I now stick with the latter - at much less cost.

And no, I don't have shares in Comma, by which I swear!


To understand the issues you must understand the oils; in simple terms there are three main types.

Mineral: this is where the oil itself is mineral derived and accounts for around 60% of the product, the rest is addatives, this is also naturally derived from mineral or non synthetic bases.

Semi Synthetic or synthetically fortified: This has a mineral oil content of around 45% mineral oil, some of the additives are manufactured synthetically, some from mineral sources.

Full synthetic: this is as semi synthetic but all the additives are manufactured synthetically.

All oils contain mineral oil, the additives work to ensure the oil stays in grade for longer time spans or higher mileages, it is the additives simply which protect the lubricating mineral content of the oil. Mineral oils have a thicker film, this is suitable for older engines which have larger internal clearances due to their materials and manufacturing capabilities of the time.

Semi synthetics have a slightly thinner film thickness, the additives ensure the oil film remains thick enough to work in older engines, but offer a longer working cycle due to some of the additives being synthetically derived. Semis can be used as a replacement for mineral oils in most applications, and for all Daihatsu models. On some older models it would be wise to change the oil filter slightly earlier as semis have a better cleansing action, usually the first two oil changes would suffice.

Full synthetics use a far thinner base oil, yet have total synthetic additives, these are only suitable for modern engines which are manufactured to modern standards. These modern engines have considerably smaller internal clearances, yet need an oil to work for longer oil change intervals, and work within a far larger temperature range, usually far hotter than older designed engines. These oils also have to clean engines far more efficiently to comply with stringent emissions requirements, and not mist as old oils do as modern cars are equipped with catalytic converters which would be damaged.

Changing to a fully synthetic oil on modern engines is fine, but remember to change the oil filter earlier for the first two changes due to the aggressive cleaning or detergent action. After this everything should be fine as most contaminants should have been cleaned away.

Do not use full synthetic in Fourtrak, F50 or F20 engines, the large clearances cause a condition called bore scuffing, this is easily explained as rubbing an angle grinder down the walls of the cylinders. Use only mineral or semis in these engines, particularly if a mix of vegetable oils is used with the engine.

Synthetic Oil

Below is a statement on the benefits of synthetics published by a leading oil company (not Comma) - just to put us, Joe Public, in our place!

The best thing to do of course is to start with the vehicle manufacturer's recommendations, followed by the oil companies' recommendations. The 'Application Guide' on the Comma Oils website, is very useful as Comma provide excellent oils at sensible prices, being a subsidiary of Exxon (Esso..including Mobil).

"Recent improvements in internal combustion engine design have led to higher performance requirements from lubricants, to such an extent that these requirements are stretching beyond the capabilities of traditional mineral lubricants in areas such as resistance to thermal and oxidative degradation, deposit and sludge control and control of volatility.
The most commonly used synthetics are PAO (Poly Alpha Olefins) and esters with some companies using the inferior XHVI bases.
Synthetics offer demonstrable benefits in all areas of lubrication (except seal compatibility) over traditional mineral oils, these benefits far outweighing the cost disadvantage.
Synthetics are manufactured chemically to give tailor-made molecular structures without undesirable contaminants, offering unique performance levels.
Features and benefits include:

* Improved film strength - better anti-wear capabilities, up
to 650% higher than mineral oils
* Better viscosity control - improved engine protection at
high temperatures
* Improved low temperature performance - improved protection
at the critical cold start period
* Better heat transfer capabilities - keeping engines cooler
* Superior oxidation stability - particularly with respect to
to cleanliness and oil thickening
* Higher polarity offering improved wear protection
* Higher natural VI (Viscosity Index) - requiring less
additive that has a potential to 'go-off’
* Improved volatility control - reducing oil consumption and
longer ‘stay in grade’
* Higher shear stability - greater resistance to thinning
with use
* Improved low temperature performance - flows more easily
at cold temperatures
* Lower pour points and lower borderline pumping
temperatures lubricating faster at low temperatures and
start up, also reducing start up loads and stresses
* Reduced coefficients of friction reducing power loss and
improving fuel economy

In short longer engine life cleaner engines with more power/ better fuel economy".

YRV Tony

YRV Tony


Re the above post.

"Recent improvements in internal combustion engine design". HOW recent !!!!!,

I do not think the engine in my F75 '91 Fourtrak TDX fits this category, these engines were built with global use in mind,for all sorts of applications, to run even on poor quality fuel and lubrication, very tolerant of the lack of servicing, ie oil/filter changes. Not designed or machined to use a Fully Synthetic Oil.

I DO use a Semi Synthetic Oil, as I often run a New Rapeseed Oil/Diesel fuel mix, as per the price differental, because I understand it helps remove the possible harmful residues from combustion.

Fully Synthetic Oil are not the answer for all. If your handbook says mineral oil,its before engine design took on the tighter tolerances that are now found in newer engine design.

Edward (ews) '92 Fourtrak 2.8 TDX

Not quite...

There's a few things that need to be corrected here. Not sure what your background is Assassin, but I think you've been had by some clever marketing. I've worked in the industry for a while and know most of the companies who manufacture synthetics from the inside...

First big mistake:
Synthetic is NOT made from mineral oil. They are actually a variety of "manufactured" as synthetic base oils as opposed to "derived from" anything else. Most common are polyalphaolefins (PAOs) and some esters. I'm not gonna go into boring details here, but google and find out more on this...

Second mistake:
When you use the terms "thicker" and "thinner", you can only be referring to viscosity. Viscosity is the RESISTANCE to flow. In other words, the higher the viscosity number, the thicker the fluid. Eg. molten lava or honey has higher viscosity, water has lower viscosity. This is a property that is totally independent of whether the oil is a mineral or a synthetic oil. You can get a thick/high viscosity synthetic oil as well as a thin/ low viscosity mineral oil. So the ratings u get on oil like 5W 30 refer to the viscosity numbers when cold and when warm. 5 is the "winter" or cold start viscosity, and is thinner because u dont want lots of resistance when u start an engine cold. However, the 30 is a higher viscosity once the engine warms up when u want the oil a bit thicker to lubricate and cool effectively. This change in viscosity with temperature is known as the viscosity index (VI). An interesting point to note here is that with the advent of fully synthetic oils the original scale of 0-100 had to be extended to over 350 to take into account just how superior they are to mineral oils.

Third Mistake:
You cannot really class additives as synthetic or non-synthetic (??). They are just chemicals/compounds that are added to the base oil in order to perform functions such as reducing foaming, corrosion, wear, and also to clean the surfaces. There are lots others, but once again I am not gonna go into massive detail, they are on the internet - just google. These functions are supplementary functions to the main 2 functions which are lubrication and cooling. Additives are typically less than 5% of the oil by volume, and only in extreme cases for very specific purposes up to a maximum of 20% otherwise they will affect the 2 main properties of lubrication and cooling.

Just a bit more info for interest...
One major disadvantage when using mineral oil is the eventual buildup of "varnish/sludge/gunk" or whatever u wanna call it. However this is mostly considered bad in newer engines. On lots of older engines of engines with bigger clearances, the varnish actually "fills the gap". Its NOT the oil. The reason you get scuffing on the walls of the cylinders is because the synthetic oil's cleaning action removed the varnish that has been deposited and burnt over time onto the walls. This aids compression and many times you may switch over to synthetic and gets smoking and rattles on startup as the varnish that was preventing that from happening before has now been cleaned away.
The important thing to realise from that is that it is NOT the synthetic oil that is thinner or substandard in this case. Because if u switch back to mineral oil, it will still do the same thing for a while until eventually (if not permanently or critically damaged) it will re-varnish the surface and raise compression or "fill the gap" again.

Just out of curiosity Assassin, when you say vegetable oils, do you mean as fuel?

Just call me G... Wink
2004 Yellow YRV Turbo

Go Juice (Fuel)

Vegetable oil.

Yes, 'assassin' refers to Veg Oil as FUEL. (quite legal if you follow the C + E rules) I and many other Fourtrak owners use straight veg oil or a veg oil/diesel mix. I run a 50/50 mix of New Rapeseed Oil and Diesel, or straight Diesel, as to the price differental per Litre.

Edward (ews) '92 Fourtrak 2.8 TDX

SO why use Fully Synthetic, in an engine NOT designed to use it.

SO why use Fully Synthetic OIl in an engine NOT designed TO USE it. In a engine THAT is designed to factor in all these possible aspects that you mention.

Emmission Tests on my '91 Fourtrak
Test limit- 3.00 l/m

2007- 2.36 l/m (multi grade mineral oil)
2008- 0.91 l/m (semi synthetic oil)
2009- 0.75 l/m (semi synthetic oil)
Same model/brand test machine, same tester.

This engine HAS NOT had any work on the Injector pump or Injectors, they are as original, as is the rest of the engine apart from the timing belt and tappet adjustment.

Semi Synthetic Oil, has seemed to have improved my trucks emmission values, and less oil is burnt in cumbustion contributing to lower smoke/particle emmissions

If an engine was NOT designed to use Fully Synthetic Oil it should NOT be used, the tolerances are designed with the lubrication available AT the time of manufacture.

"if not permanently or critically damaged)" What, for switching back to using the oil original recomened for this engine .( any engine of this era)


"Just a bit more info for interest...
One major disadvantage when using mineral oil is the eventual buildup of "varnish/sludge/gunk" or whatever u wanna call it. However this is mostly considered bad in newer engines. On lots of older engines of engines with bigger clearances, the varnish actually "fills the gap". Its NOT the oil. The reason you get scuffing on the walls of the cylinders is because the synthetic oil's cleaning action removed the varnish that has been deposited and burnt over time onto the walls. This aids compression and many times you may switch over to synthetic and gets smoking and rattles on startup as the varnish that was preventing that from happening before has now been cleaned away.
The important thing to realise from that is that it is NOT the synthetic oil that is thinner or substandard in this case. Because if u switch back to mineral oil, it will still do the same thing for a while until eventually (if not permanently or critically damaged) it will re-varnish the surface and raise compression or "fill the gap" again."

Edward (ews) '92 Fourtrak 2.8 TDX

misconception about design standards & specs

The quick answer to that is that the engine wasnt designed for mineral oil specifically. It would normally state a viscosity rating and possibly a quality standard. If there is a potential for seals or rubbers to be affected by synthetic oils, then it would state it (some older cars wont because synthetics werent that common not too long ago). But otherwise, the "design standards" are the minimum requirements.

So a 5w40 mineral oil and a 5w40 synthetic oil will do the same job as a lubricant and coolant. But for a toyota aygo, where the main customer is a budget concerned customer, and the car is not a performance car that is pushed to the limits of compression and revs, there is no sense in quadrupling service costs. However, an M3 which might have the same viscosity range (or might not, just a hypothetical case) will specify the need to use synthetic because of the other factors around it (that i mentioned in the earlier post).

To put it in another example, spark plugs... for a common compact or budget car, most will specify normal NGKs or denso using eg. platinum etc... BUT its a fact that the iridiums will perform better. But not everyone will pay 4 times the price for the benefits.

Bottom line is that most people just buy a car to drive for a few years till they trade up. All they use it for is to work and back and some family drives. Why would they spend that much extra when there is no feasible benefit for them? But then u get car enthusiasts who will squeeze the last bit of quality out of every facet of the car to get performance and longevity among other things.

Some people would spend oodles on the best 7-step car wash, wax and polish range, others will buy cheap crap from the corner shop or even use dishwashing liquid. Both will make the car clean... etc...

Just call me G... Wink
2004 Yellow YRV Turbo

Dish washing liquid

I have used this for the life time of my truck and it still looks good, just a good wax afterwards, dont knock it !!!!!!. Even with a pearl paint finish !!!!!. all your buying is a surfactant, why pay for hype in a soap product, to remove old wax and crud !!!!! O'h nearly forgot, I use cheap car spray wax from the pound shop, far better than expensive waxes, watch the crap wipe off and the water just repeled off. Cost is not a facter. Marketing hype again, results count!!!!!.

Edward (ews) '92 Fourtrak 2.8 TDX

agreeing or disagreeing?

Cost is not a factor?
marketing hype?
I cant figure it out, are u agreeing with me or not?

Just call me G... Wink
2004 Yellow YRV Turbo

Where are you comming from !!!!

Your saying that the F75 Fourtrak engine was not designed for mineral oil !!!!!! Read the owners hand book !! and remenber this is a global engine, for use in ALL parts of the world.

"The quick answer to that is that the engine wasnt designed for mineral oil specifically"

Edward (ews) '92 Fourtrak 2.8 TDX

Hi EWS You are right, the


You are right, the engine was designed for mineral oil as it was an old design (early 50's) and synthetic oils were still in the early stages of development, so mineral was all that was available worldwide, or vegetable derived in some third world countries.


And just whats your background !!!!!!,

Edward (ews) '92 Fourtrak 2.8 TDX

Mistakes, interesting. My

Mistakes, interesting.

My posts are simplistic to ensure most, if not all posters understand them.

The base oils you refer to are those specifically manufactured for the use of a variety of additives by differing manufacturers, as you should know these vary.

I was not referring to viscosity, i was referring to engine design and manufacture and the tolerances involved such as bore or bearing clearances to which they were designed to be manufactured. This was not viscosity it was film thicknesses required when comparing old engines to new engines, and the differing characteristics required by the oils for which they were designed to run on. I certainly need no explanations about viscosity which is a very broad term and notoriously variable, particularly when i work to Saybolt Universal Seconds and Centistokes. I did find your explanation of the VI being extended, particularly interesting as neither the SUS or centistokes needed extending to accommodate them.
The paradox here is that both the old and new scales totally contradict themselves with their viscosity ratings, a point not lost on the physicists and engineering scientists, which is why they are notoriously inaccurate.

The additives packages are classified as synthetically derived, mineral derived, or even vegetable derived but only the classic oils use vegetable derived additives. This where much confusion arises, it is the terminology used by differing base oil suppliers and manufacturers marketing departments which hype aspects of the product, and the manipulation of the marketing people.
To answer your other point, no i am not conned by clever marketing.
Viscosity improver packages can contain many components of any derivation, not just single chemical compounds, this is why they are so much better with boundary lubrication and electrochemical bonding of both metallic and oil particles, particularly during cold starts.

Varnishing occurs with all oil types, it is the superior dry plating elements which provide superior lubrication with synthetics, and the additional pressure characteristics which allow these compressed particulates to readhere. With mineral and synthetics it is simply a differing type of varnishing, by the use of calcium sulphonates instead of the magnesium sulphonates and the various waxes and silicone based additives.

If additives only account for 5% by volume, this makes every technical data sheet from every oil manufacturer a total fabrication, i refer to Total, Shell, BP, Castrol, Millers, and many others i have. Their polymeric content is usually highr than 5%, this being only a small content of the additive package.

Full synthetic oils are thinner, but provide better lubrication and cooling characteristics because they are thinner, it is this heat dissipation which makes them superior. This is why the latest engines use higher flow lower pressure oil pumps, this ensures the oil is circulated quickly to remove heatquicker, and can dissipate it quicker when returned to the sump where the coolest airflow is achieved. Combine this with the modern anti friction plating on many components, and the cera-metalide components and we have far superior lubrication from extremely tight tolerances.

As for experience, well i did design the taper bore systems on formula 1 engines, now used by all teams, and worked on the specialised lubrication required for these systems. I introduced the synthetic oils into British Coal for use in hostile environments, as well as designing and constructing the oils from coal plant and equipment to ensure the scientists got what they wanted from this effective process.
In addition i have worked for nearly two decades (in my own time) on a range of bio-fuels projects for a number of my professional institutions, and worked with a number of the worlds best lubrication scientists and engineers to develop specialist lubricants for these systems.

So i do not need to google anything, but please tell me when you know about the latest planshterising lubricants, and you cannot google that because it is still in the early development stages.

Keep it on the subject...

Assassin, I said it earlier and I’ll say it again… Synthetic oil is not derived from mineral/crude oil. It was developed through a chemical process known as the Fischer-Tropsch process with raw materials like methane, carbon monoxide, and carbon dioxide. Variations of this obviously existed and still are in constant development. If you’re interested, this process was developed by Germany in WWII, because of the lack of crude oil resources.

If you go according to API classification:
- Group I and II - these are mineral oils derived from crude oil
- Group III - this is a highly refined mineral oil made through hydrocracking..
- Group IV - these are true synthetic oils, known as Polyalphaolefin (PAO).
- Group V - these are synthetic stocks other than PAO's and include esters and other compounds.
As for your throwing of SUS and centistokes into the discussion, well as you say they are not used interchangeably in a reliable way here, so why bring them up? Rather stick with nominal viscosity as used internationally for engine oil, and discuss using the ratings used by international manufacturers eg 5w-40 or 0w-30, where the w (regarded as the winter/cold temperature) shows the nominal viscosity when the car is started or in very cold conditions, and the 2nd number gives the nominal viscosity at 100degrees C.
And in your 3rd explanation, you again say that synthetic oils are thinner. No fancy words necessary here, you are wrong. A 0w-40 cheap mineral oil and an expensive fully synthetic 0w-40 will be exactly the same “thin-ness” at both 10degrees and 100degrees. If they don’t they would be breaking the law by not conforming to the quality specs stated on the bottle. Sure you can throw out some more fancy words (which are completely outside of what this discussion is about by the way) regarding the atomic/molecular level of tribology, but that is not what we are talking about here. This is about mineral vs synthetic vs semi-synthetic and the incorrect facts stated about their use in cars.
I don’t need to know about planshterising lubricants, because (as with all things in development) if I cannot use it or have no access to it, it cannot help me. I am not a design engineer, and I never will be. I am the guy that gets called when the design engineer didn’t take something into account and it needs to get sorted after being in operation. It is part of my job to learn and stay updated with current available technology instead of blowing my own trumpet about a “tapered bore” I worked on in the late eighties that would be catastrophic if applied to any engines besides F1 engines (that are nowhere near as successful as u made them out to be). I’m glad you know the world’s best lubrication scientists and engineers – please ask them about how u possibly get synthetic oils from mineral oils (??)

Just call me G... Wink
2004 Yellow YRV Turbo

On the subject

When we refer to thickness, it is not viscosity it is film thickness, the synthetics being a considerably thinner film thickness than minerals of the same viscosity. Viscosity is a totally different issue, the SAE/API standards are notoriously unreliable, but simply a marker for vehicular or other applications. This is why Centistokes and SUS measurements are used by lubricant developers and it is then classified according to its SAE/API rating to make it understandable to the layman.

You seem to be unable to understand the difference between the base oil (mineral) and base stock (synthetic) so it is at cross purposes.
Just for your information it was not a german invention, it was originally developed in Norway, the Germans adopted and developed the synthetic technology. Development was for many reasons, lack of oil supplies being only one, the main reason for its development was for the prototype war machinery, the jet engine being the most popular application. This being a common misconception shared by many.

Why resort to such tactics, the taper bore system was so unsuccessful it was adopted, and is still used by all F1 teams, and a great many other piston engined applications requiring high power applications. I am not blowing my own trumpet, simply highlating a case which saw lubrication as the most problematic factor in its introduction. This case led to the introduction of many of the original cera derived anti friction coatings, this now having filtered down to production road engines.

I have been clear that all engine oils, semis and fully synthetic compounds are comprised of mineral oil; which they are. You buy a can of oil, this is a compound of many substances, you do not buy the ingredients and blend your own. The synthetic compounds simply protect and enhance the mineral content, perhaps you should read my posts properly.
Thinner film means it retains and improves upon the minerals considerably, but can be better used with modern engine technology. Coefficient of friction for example, lower oil pressures, but improved flow rates mean better cooling and removal of deposits in suspension. Enter the anti friction coatings now used by many engine manufacturers, a problem for mineral oils which destroy them, synthetics are designed to work with them.

Many engines cannot use full synthetics, this is why semis were developed, they fulfilled the role of minerals in the greatest majority of cases, but not all, and proved to be a better option due to the tightening of vehicular emissions legislation. These could be described as a stopgap lubricant. Full synthetics were predominantly used or developed in conjunction with vehicle manufacturers for specific requirements to ease the changeover to them progressively, while ensuring an option for older vehicles.

No i do not bother with fancy words as there are many people who contribute to this site, they are all of differing intelligence levels and capabilities, this means keeping it understandable to them.

Incidentally, Wilko's do a semi synthetic oil under their own brand name for £8.49 for 5 litres so you can use them at this price, and check their specs.

your own standards

1 thing is for certain, you definitely don't subscribe to many internationally recognised or commonly used scales of measure. Must make it a nightmare whenever you buy anything. Even a trip to buy groceries must be a traumatic experience. I respect that you have your own train of thought for oils as well. But its unfair to say things like "when WE refer to thickness..." because it is most definitely not shared with the vast majority of the technical population. The thickness of any fluid is the resistance to flow. If you add the word "film" before it, then only does it mean the film thickness which is a measure of a different dimension.

Which brings it all back to WHY synthetic oils have a thinner film thickness - they are chemically built/derived. And yes, some constituents of synthetic oils are derived from CRUDE OILS, note - not to be mistaken for CONVENTIONAL (Group 1) BASE STOCK MINERAL OILS (Yes I do know the difference between base oils and stocks which is 1 of the issues I had with your original explanation in the first place). Mineral oils are derived from crude oils, but synthetic oils are NOT derived from mineral oils. That is the REAL misconception.

Now you may disagree with what I say but these are industry facts:
Check on all these websites and they clearly state the difference on where mineral vs synthetic oils are derived. I don't have time to check all the other oil companies but I'm sure anyone on here can do the same for any other brand and find the same results - why? - because it's the true facts...

As for where it comes from, I never said the Germans INVENTED it, if you read my post, I said "Variations of this obviously existed and still are in constant development." This particular process (invented in the 1920's by Franz Fischer and Hans Tropsch who were German researchers) was the one that during WW2 made it viable to mass produce both synthetic fuel and oil.

I emailed an old colleague of mine who previously worked in engine development (both in and out of F1 and now in A1) for the last 35 years, and I am waiting on a response about taper bore systems. I admit my ignorance on that subject, because as far as I know taper is bad - taper on cylinders is due to wear, on pistons - but that is for dimensional expansion with temperature to maintain sealing geometry, and with taper bore valves - unless you were around in the early 19th century, you didn't invent that. So I am actually (genuinely) interested in what the f1 system does.

I am due for a service on one of my cars, and the wilko's sounds like good value. Where can I buy that?

Just call me G... Wink
2004 Yellow YRV Turbo

Really cant understand all

Really cant understand all this talk about film thickness with mineral, semi and full synthtics.

The viscosity grade ditates the film thickness, We know that older engine designs or high mileage engines will be better off using a 40 or often a 50 (even 60) grade, but it makes no difference if it's mineral or full synthetic.

These days the synthetics widly available in the high street only go up to a 40 grade, the semi's and mineral 40 or 50 grade.

However full synthtics are available in all grades...0w30, 0w40, 5w40, 10w40, 15w40, 5w50, 10w50, 10w60, (Redline even do a 20w 50 full syn)

Synthetics ability to provide superior protection cannot be dismissed,

With my old Rotary NSU Ro80 I restored in the early 1990's, The worn out engine was rebuilt with new apex seals, and the car was then ran on the original Mobil 1(a 5w 50).
In the 1970's with the oils available the Ro80 used to eat engines at a rate of 20,000-30,000 miles.

With Mobil 1, I clocked up well over 60,000 miles (before selling it) and compression was still good.

IMO, A heavy grade full synthetic like a 10w60 or 10w50/5w50 would be ideal for a fourtrak (or any other older design/high mileage engine) if you so desired.

Infact the latest Mobil 1 10w60 "extended life" (replaces Motorsport 15w 50) states it is for the older engine or race use.

Of course if you try using a fully syn 0w30,0w40/5w40 in a old engine that prefers a 15w50, then you can expect problems.

It's all in the grade, not the oil type, The additives used in all oils are compatable, (mineral, semi and full syn) and any oil blended in the last 20 years or so can be mixed without issues.

Any old myths/stories about synthetics causing oil leaks, well that can only be down the the poor choice of grade.

Why semi-synthetics are a ripoff... (financially)

well... i am not sure i said it clearly enough what i meant when i said this.
Mineral oils are comparatively cheap. Fully synthetics are comparatively expensive.
Here is just an example of 3 oils from the same manufacturer (i'm sure there are many other variations and some exceptions to the rule but this is what is mostly the case):

Castrol Mineral Oil GTX 4l - 16.99
Castrol Part-Synthetic Magnatec 10w40 4l - 29.99
Castrol Fully Synthetic Magnatec 0w30 / 10w60 4l - 41.99

I tried to get the average prices from halfords, ebay, amazon etc, but obviously havent checked everywhere, just scanned around briefly to make the point.

Now mineral oil still makes up the bulk of semi-synthetic. You can phone ANY oil manufacturer and ask them the percentage of synthetic oil in their semi-synthetics and I can guarantee u that almost all will be less than 15% and the MOST would be maybe 25-30% synthetic.

But for some reason they charge u about DOUBLE compared to mineral oil for the same VI and spec? You may get a fractionally better cleaning action and some minor improvements in characteristics, but how about this:
Rather buy a 4l mineral and a 1l fully synthetic and mix them yourself. You will get a better ratio, you will pay less AND you will have some left over for top-ups!

Its a RIP-OFF! People fall for it everyday cos they see "semi-SYNTHETIC" and think they are getting far superior performance.

Your example about visible wear on the rocker faces? I am a bit confused... did the fully synthetic oil "remove" the wear or stop the wear? And how did u measure the wear rate? How long was that oil in for? Or how old is the engine? A common problem (though u said that its not the case on yours) and its not the oil itself, its the sludge/varnish that sometimes builds up on the oil feed lines and reduces the flow making the lubrication and cooling insufficient and wear and damage results...

Fully synthetic oils are able to contain higher amounts of "anti-wear" additives which keep parts separated more uniformly and able to protect better.

Your 350cc twin will never really test the extremes of a fully synthetic oil. And a SS will work fine from now. My point is still that for the spec of the SS oil its a rip-off.

Just call me G... Wink
2004 Yellow YRV Turbo

350cc twin !!!!!!

Are we now talking moterbikes !!!!!!! ON this site ????????

Edward (ews) '92 Fourtrak 2.8 TDX


Clearly you have misunderstood my post so i will explain simply.

Oil is a manufactured complex chemical composition containing mineral oil to lubricate the engine, and a lot of other additives to perform other tasks. The base oil is always mineral, it is the additives which are synthetically manufactured, not the base oil, and these additives are the majority of what you actually get when you buy a container of oil.

Below i will list this chemical composition, this is for an actual semi synthetic oil currently on sale for diesel engines.

Base oil 45% Mineral


VI (viscosity improvers) 17%
Anti-scuff Reagents 3%
Anti-oxidants 5%
Corrosion inhibitors 6%
Detergents 5%
Dispersants 5%
Anti-sludge 6%
Anti-wear 6%
Other additives 2%

As we can see it is the additives which make up the majority of the oil, some or all of these additives can be synthetically manufactured, the base oil is always mineral oil. Where they quote the synthetic oil content as synthetic, they are actually qouting the percentage of additives which are synthetically manufactured as no oil is actually synthetic it is mineral.

These additives used to be all mineral based in years gone by, but the mass production of some of these additives has made them cheaper or similarly priced to manufacture. Modern engines have longer service intervals, this means the older oils cannot maintain their integrity and protect the engine for the extended service intervals required by modern engines.
Environmental emissions is another aspect, older mineral oils produced a vapour, modern catalytic converter equipped engines would damage their cats if this vapour entered the cat.
Other factors enter the equation, these being engines which run far hotter in modern vehicles, higher loadings on smaller components, and the lead being removed from petrol.

I hope this explains the situation.

Semi-Synthetic oil

Blood everywhere when experts clash!

As a retired technologist myself I enjoyed the experts' exchange of facts and views.

Many years ago I was staying in a hotel on a business trip and had struck up a breakfast table relationship with another engine enthusiast. We were desperately trying to impress each other with our knowledge of cam profiles when I broke off to apologise to a third person at our table for being boring. "Oh" he said "I don't find it boring, I'm the chief engineer of BRM". Arrrgggg!

Anyway, it is indisputable that changing from mineral oil to semi-synthetic oil drastically reduced the rate of rocker wear in my motorbike engine and compared very well with f-s for that purpose.

For my bike, Comma's semi-synthetic oil is definitely not a rip-off and is priced at less than some mineral oils.

No more blood please!

YRV Tony

YRV Tony

peeing in corners

Well for one thing, I don't think there's any blood... a good debate is always healthy and for the most part, the audience benefits.
If I could find semi-synthetic oil cheaper than it's mineral oil counterpart of the same reputable brand, I would also buy it. So we don't disagree on that, when i said rip-off, i am referring purely to the cost difference vs what u actually get.

What i don't like is it seems that cos i'm the new guy, there are certain regulars here that feel the need to pee in all the corners I'm standing on to assert their dominance over their territories. I cannot spend as much time on forums and answer/reply to every little comment as I work long hours, but will try to discuss whatever.

Just call me G... Wink
2004 Yellow YRV Turbo


Not so. I found your comments in a previous post ( To Flush or Not to Flush), re the degradation of NEW oil ( whether it be Synthetic or Mineral) from the residual Flushing Oil most interesting. As there is no way that draining the oil pan will remove ALL the flushing/old oil mix, I asumme even more is left if a suction device is used, plus what is left within the internal parts. I understand what you say about water/oil contamination and the need to flush the oil/water mix from within the engine internals.

I would and probably others would find discusion of this idea interesting. I think you should bring this up as a post in its own right. As it would be of interest to ALL Diahatsu owners, no matter what type of oil we use, or cars/trucks or age. and while were on additives, lets have some discusion on Oil additives, engine /gearbox and Fuel additives, injector cleaners and the like. Possibly LuRcH could place them in a sub section of the site.

Edward (ews)

Edward (ews) '92 Fourtrak 2.8 TDX

Film thickness and viscosity

Film thickness and viscosity are totally different things. Viscosity indices were introduces for motor applications simply for ease of understanding, and are a small component of SUS or centistokes measurements.
In basic terms, an easy way for the laymen to understand oil requirements.

Film thickness is the actual thickness or film of oil on a surface, and this dictates many things. Minerals had thick films, semis thinner films, but could replicate minerals, and full synthetics have even thinner film thicknesses.

Film thickness dictates internal clearances in an engine, old engines had large internal clearances for minerals to provide effective lubrication. Technology advances, and internal clearances got tighter due to better repeated machining capabilities. Engines run hotter due to better combustion, and this necessitated a better oil which would work with tighter clearances, and remove heat better.

This led to the synthetic oils becoming adopted, once they were very specialised oils only used by the best engines. Now technology had advanced and filtered down, so have synthetic oils.

Hope you understand this.

Sorry no i

Sorry no i don't. And I don't agree. Viscosity and film thickness are not totally different things.

"Regardless of how film thickness is measured, It is a function of viscosity, velocity and load"

"As viscosity or velocity increases, so does film thickness"

Final paragraph page 11