Try cleaning the PCV valve, it may be stuck open allowing fuel to get sucked into the crankcase breather. Next, pull the carburetor float bowl off and check if the fuel float valve is sticking open. The PCV valve is behind a small cover with a tube that goes to the carburetor intake, this is the only direct path for the fuel to get into the crankcase.

It may also have a bad fuel cutoff solenoid in the float bowl if equipped - http://outdoorpowerinfo.com/repairs/imag...

When the engine is not running, the needle and seat in the carburetor is leaking fuel into the cylinder past the piston rings and end gaps to the crankcase. You have two choices: Install an inexpensive fuel shutoff valve or replace the needle and seat and check the float level.

Is there a petcock between fuel tank and carb? Was it shut off during storeage? If gas is allowed to flow into carb during storage and float/needle and seat stuck open then gass would or could get in crankcase. It's a good idea to shut off fuel to carb and let engine run until fuel is gone. This will keep sludge and varnish from building up in carb. Particularly during storage. As fuel evaporates from carb the heavy gunk is left behing and causes carb and float to clog up. This causes float to stick open draining fuel into combustion chamber then down into crankcase.

My guess would be check and clean carb.

I'm trying to understand how there is gas in the crankcase. Unless you put gas there, and even if you did there should not be gas there. This would have nothing to do with engine or carberator going by what you said. The best thing for you is have it looked at by someone who knows how to repair the problem as your understanding of how it runs does not sound good.

Try turning the fuel tap to the off position when the mower is not in use.

I have a four stroke 15.5 HP OHV riding mower engine that has gas in the crankcase oil. I have a gravity feel fuel tank with no fuel pump. Do I have a carburetor or internal engine problem?