Why would there be oil in your radiator?
Odds are you have blown a head gasket and the antifreeze and oil flow share the same gasket. As the gasket wears, the antifreeze mixes into the oil and vice versa! I suggest replacing your current antifreeze and do a full flush. Test it for a few days and test if you have oil again. A tell tell sign of a blown head gasket is black smoke (burning oil) or white smoke (burning antifreeze ) coming out the exhaust.