Head gasket blown between cylinder and pushrod chamber. Running it on sidehill (Carb up) combustion chamber is pulling in oil that returning to crankcase. Do a leakdown test. Briggs OHV's model 28,31 and 33's this is quite common. And whoever replaces the gasket make sure the HEAD IS RESURFACED. Not uncommon to see heads .008-.012 warped in area gasket is blown out.