I hope someone has the answer. I noticed oil floating on the top surface of the expansion tank after I has the oil changed. The first thing I thought was some stupid kid put oil in the radiator. I flushed the system and had the dealer test to see if there was CO present in the radiator, just in case. There wasn't so we ruled out blown Head gasket. I didn't see any more oil collecting in the tank right away. But over time a small amount will collect again. All hoses have replaced and the cooling system has been flushed out twice to eliminate any residue. What could it be....Someone said the radiator probablly has a leak and when the car is under a load the pressure on the oil side leaks into the coolant. FYI there is no trace of coolant in the oil. Is this true or does anyone have the answer to this million dollar question, last thing I want to do is shell out money on something that will not help.