Verified answer

If it has a mechanical fuel lift pump attached to the engine then the diaphragm in it is probably leaking, this will allow fuel into the oil. If a lot of fuel is getting into the oil it will dilute it so that it's not an effective lubricant, which will cause the engine to wear more quickly, or even seize up in extreme cases. With petrol it's also a fire risk.

Diesels also can do this due to leaky fuel lines to the injectors if they're under the rocker cover.

If it has a carburettor which is leaking fuel into the inlet manifold overnight then this will get into the cylinders, and then past the piston rings into the oil, this is more common with motorbikes because the fuel tank is above the engine, which means that the whole contents of the tank can end up in the engine if the carburettor float valve is worn because gravity will keep supplying more fuel.

change the oil if you know theres gas in it.... if your wondering why its in there you might wanna get some professional to look at it 0.o