Are you sure it is oil? If the coolant system has been exposed to air from a bad radiator cap or the coolant has never been serviced it can get a brown "goo" that looks like oil.
If however you are sure it is an oil, what color is it. If red in color it is a bad transmission cooler, located in the radiator. For that you will want to check the transmission fluid to see if it has coolant in it. If it does get a flush done and hope it was caught in time, coolant does bad things in a transmission.
Also you will need to change the radiator and flush the cooling system with a good detergent flush to remove the oil. Then re-fill with fresh dexcool.
If the oil is a dark color you should start with a pressure test on the cooling system. There are very limited ways that oil can get into the coolant. A bad head gasket is one. A pressure test would give you an idea of the issue.
Saturday, September 2nd, 2017 AT 12:34 AM