Oil in the overflow tank? on 1998 GMC 1500 Pickup

I bought a 98 pickup and it has been sitting for about 6 mos. I checked all the fluids and it had about a 1/2" of oil floating in the overflow cooland tank. The motor runs great and the radiator water doesn't look bad. Where could this have come from? I cleaned out the tank and hope that it won't do it again. Any other ideas? Thanks-K

Asked by for the 1998 GMC 1500 Pickup
GM vehicles have had problems with Dexcool coolant eating through their intake manifold gaskets and then leaking oil in to their coolant. If it persists, I would think about replacing the intake manifold gasket and possibly the whole manifold, if it is pitted, ASAP.
This pickup had green coolant in it. Did the 5.7 have this problem too? I thought it was a V6 problem. Thanks-K