Gear oil viscosity Fourtrak F78


Can 4Trak owners please advise - Gear selection becomming more scratchy as oil warms up. Local 'experts' recommend using standard SAE20W-40 type motor oil instead of prescribed SAE 75 - 85 since temperature rarely drops below 10 deg Cent. in South Africa. I'm unsure if this is advisable. I've added 'Prolong' to boxes to evaluate, found negligeable improvement. Checked mainshaft nut on back of Transfer - still tight. How safe would it be to decrease the viscosity for reducing the scratching gear change?

20-40 .... NO WAY

IMHO, the gearboxes on these beasts are the weakest link in a very strong chain. Actually, they're still pretty good, but when you are expecting the engine and vehicle to do 300,000km with ease, the gearbox shows signs of getting old first.

Stick with the recommended 75W-80 semi-synthetic oil and add a molybdenum disulphide based friction reducing agent ('Molyslip' in the UK).

The recommended gear oil (and transfer box too) change intervals are 36000km, but I have fudged this to 50,000km. Purists here would say that I'm pushing it, but I say that the vehicle is used on the road most of the time, so isn't working too hard. That said, I'm just about to start towing a trailer loaded with a mini tractor and hedging attachment quite regularly, so I'll change the oil to cope with the extra 2000kg towing load.

Oily Subject

Do not use any engine oil in the gearboxes, transfer case, or axles; this does not have the capability to withstand the shear forces imposed on the oil, you may as well piss in the gearbox as use this.

Basically gearbox oil stands up to extreme loads as the gears contact, it is designed to form and contain an oily lubricant between the meshing gears, thinner oils would literally be crushed and pushed out leaving metal to metal contact.

Most gear oils are EP or extreme pressure, and are a synthetic oil, these are what the gearbox needs, put an additive such as Slick 50, Molyslip, or other locally available brand of additive in the boxes. Most are teflon based and will coat the gears with an additional coating, this will often build up the surface of the gears slightly and improve the characheristics of gearchanges if wear is not excessive.

Thanks for such clear

Thanks for such clear concise answers - and saving my backside! (I guess the 20-40 could be the famous African way Wink )

find a can

find a can of ep90 gear oil if i was you, its just slightly thicker for warmer temperatures, thats what i use anyhow not to mention nearly every other 4x4 gear box, its not much difference to 75/80 but its easier to get and its the same gl5 grade etc, in one of our bits of machinery we have to use ep140 now that is mega thick syrup!!!!

Full of ideas but no time to do them!!
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