Don't know how old your mower is or what kind of engine you have. There's only one way fuel can get into the crankcase (oil), that's threw the carburetor. That's assuming you have one on your engine. Inside the carb there is a needle valve that's not seating to shut off the flow of fuel. This will allow the fuel to leak past the valve and into the oil crankcase. On some models (don't know what you got) has a vacuum operated fuel pump mounted on side of engine with three fuel lines connected. If the diaphragm is bad in the pump it will allow fuel to run back into crankcase. I would replace needle valve first and also the needle valve seat.