If motor oil, a head gasket could be leaking oil into coolant.

If it is just in the radiator and is red, it is from transmission oil cooler in radiator, it has ruptured and the radiator would need to be replaced.

If coolant is in crankcase, STOP driving. Coolant will wash the oil film off main and cam bearings and cause MAJOR engine damage.

Answered by mmprince4000 (expert)
8,853 answers provided