Small Engine Troubleshooting Problems? Ask a Mechanic Now.
Gas gets in oil from a leaking needle valve in the carburetor . It leaks, and the gas floods into the air intake where the vent hose from the crankcase is located. The gas runs through the vent line into the crankcase.
What is the engine make and model? It can only leak when engine is not being used. I assume if it has a gas shutoff, you did not use it.
The needle valve and float is what controls the gas. You can rebuild it, but you may want to check the price of a carburetor too.