Those are two separate and independent closed systems. There should be no WAY that they should cross contaminate that way. The only two ways I can think of that oil got into your coolant is #1 that the head gasket was not fully torqued and the head surface has warped. #2 The other way oil could get into coolant is human error, by pouring oil directly into the radiator.

89 Raider - V6, Automatic, rear LSD, A/C Retrofit, Cruise Control Retrofit.