I had a similar problem with my 2003 Silverado 1500 vortec 4.3L V-6. Turned out to be a blown head gasket. I was also losing coolant but it didn't show up in the oil. That turned out to be the water pump. The bad water pump may have caused overheating which may be why the head gasket blew. I was told by an old schooler that my year range GM pickups had a problem with head gaskets being manufactured too thin and blowing between 80-100K.