If there's oil in your coolant, I would suspect the headgasket, as well. I would do a compression check, although you can have good compression and still have a blown headgasket. Are you displacing any coolant into the reservoir at all? That's what my car did under boost when the h/g was blown. boost and combustion pressure would push the coolant out of the cooling system and into the reservoir. When enough of it was displaced, I would overheat.