Don't pull the heads off yet.
When you get a chance, do a leak down test. If you don't have a leak down tester, use a compression checker hose and apply air pressure to the cylinder with the valves closed, remove the radiator cap before applying the air pressure. If air is escaping from the open radiator, then you possibly have a cracked or warped head, or a blown head gasket, or a cracked or warped block.
If you have no leaks there, then there is a very good possibility that an intake gasket is not sealing and sucking oil in from the lifter valley. I hope this helps.