I checked the water level in my radiator and noticed a thin film of black oil on top of the water. Checked the water while running the engine without the radiator cap. Some air bubbles the first half minute, then no more. Engine oil level ok, oil looks normal not milky. The engine runs as always, no loss of power. Anybody has an idea ? I was thinking it could be the head gasket or a cracked engine block (hope not). Thanks for your ideas.