I have a 1993 Passat GLZ VR6 and it had oil in the antifreeze overfill tank. There is no trace of water or antifreeze in the oil though. I replaced the oil cooler, flushed the system with water untill it was clean. I drove it for about 20 miles and checked it and again there was oil in the tank. anything else i can check? flush the system couple more times?

 

Have a block test done to eliminate head gasket trouble probability.You don't mention the color of the oil or if the car is automatic.I have seen automatic trans. leak thru the radiator core since the fluid travels there for heat exchange.Also some coolants or anti-rust and water pump lubricants may leave a residue that is similar to oil.Let us know what the outcome is.Good luck