Probably as simple as you cap gasket. But your oil trap (aka oil separator box) behind the intake manifold may need replacement or cleaning. Seems that one symptom of this condition is oil pushing out of the blowby outlet on top of the engine and the oil cap. Do you also get crank case smell in your cabin when you run the heater using external air. That's another symptom. Anyway, do some searches and see what others have said about this. Good luck. John