Why do I have engine oil in my radiator

What is your question?

I have a 1980 camaro, I keep losing engine oil, its not going out the tailpipe, I can't see any leaks, but when I checked my rad, it was full of oil. The rad has a transmission line cooling the tranny and this site says that if the head gasket is leaking it will cause radiator fluid to go into the engine oil, so I was wondering if anyone has had this happen before and could dissect my problem before I sink money into fixing the wrong part