I have had gas get into the oil on an older 17.5 hp briggs engine riding mower that was gravity fed from the tank. It happened because the fuel shut off solenoid failed, and pressure from summer heat in the tank caused the carburetor to flood, and gas to fill the crankcase. The positive solution was to install a good fuel shut off valve in the line, and shut off the gas several seconds before shutting the mower down. I didn't care to replace the solenoid. I think this could happen on any mower, but less likely on a mower that has the tank below the engine, with a fuel pump, unless the tank is filled to capacity and left that way inside a hot building or out in the sun.
If substantial gas is in the crankcase, the oil should be changed twice, running for a few minutes in between, and change the filter with the second oil change.