If your car is fitted with an oil cooler, this could be leaking oil into the coolant. Try bypassing the cooler to see if oil is still getting into the coolant.
It could also be a failed headgasket which would need the cylinder head removing to replace the gasket. It's best to also have the head skimmed and pressure tested.
Whatever is the cause, you will have to flush the cooling system to remove all traces of oil. The coolant hoses may also need to be replaced as they can swell internally when contaminated with oil. Ford specify a bio-degradable liquid laundry detergent is used for flushing the cooling system.
Mike, January 2009
