I troubleshot a tractor that I had that problem with for a LONG time before checking the valve clearances, and that was the problem. Valves were not closing completely, so it was not getting full combustion, thus gas was getting into the oil.
Maybe in your situation that's not the first thing to try. What do the plugs look like? Are they fouled? It could just be running too rich and the carb may need adjustment. If the needle valve was stuck open, then you'd flood out and gas would drip from the carb if you didn't shut the gas off every time you stopped the engine. A flooded engine will let some gas into the oil. Any of these things happening to you?
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