what can cause oil in expansion tank?

had a few oil coolers go with resultant oil in the cooling system,would think about sourcing one from a scrappy as they can be very expensive,put some detergent in the cooling system and run for a day or two,drain system and repeat,fit oil cooler and give it another flush out with detergent,you may have some problems in the future with coolant hoses and refill with 50/50 a/freeze mix,very messy job and very difficult to get rid of all the contaminates in the system

cdc
December 2010