If you have oil in the water, it's almost certainly the head gasket. It's gone between a water jacket and an oilway, so the engine will run normally etc., with no sign of overheating. A head gasket can go from water jacket to cylinder ( this will cause overheating, loss of water, excess pressure in radiator hoses and water/steam from exhaust ), water jacket to oilway ( explained as above ), cylinder to cylinder ( loss of power, misfiring etc. ), or cylinder or water jacket to outside ( chuffing/hissing sound from cylinder head area, or visible loss of water from cylinder head ).With your problem, you will also have water in the oil. This will manifest itself by a mayonnaise type substance on the dipstick, or inside the oil filler cap. It may not be extreme enough to show this as yet though. Hope this helps.