1992 Cadillac Deville. Looks like there is oil in the radiator but no water in the oil.

Check the transmission oil ... oil from the transmission goes through the radiator (via heat-exchanger) to be cooled... the heat exchanger may have a leak

Thanks Tom, There are no visible leaks in the heat exchanger & transmission oil looks clean & red but a little low. The car has been sitting for 4 years without use. The fluid in the radiator looks and feels thick but smells like antifreeze. There is a tag on the radiator that says a GM approved supplement /sealant should be used with the antifeeze to protect the aluminum. Could that be it?

It may not be oil so try this ... drain the antifreeze into a clean container ... skim the top with a clean plastic cup ... cover it well (so your wife doesn't have a fit) put it in a freezer to determine if it will congeal like oil ... it's probably just a trace amount from sitting... Probably wouldn't hurt to flush the radiator put fresh antifreeze in ( some anti-rust too) and save the old antifreeze for "Topping off" if you have too

I'm gonna flush the system and add new coolant and "supplement" and see what happens. Thanks for the tips.