Small Engines (Lawn Mowers, etc.): Kohler 17.5 hp engine, riding lawn mower, hp engine

Question
Recently I cranked up my 42" riding lawn mower and it started up immediately and ran great for about 2 minutes then it started stumbling and white smoke came out of the exhaust. I thought it might be old gas so I decided to drain the gas and change the oil and filter at the same time. Once I began to drain the oil, it was very thin and smelled like it contained gasoline. I pulled the oil filter and it was the same. changed the oil, filter put new gas and gas filter on and started it up. It ran extremely good, in fact I mowed about one acre with no hesitation nor smoke. The question is: Is there any way gas can get into the oil sump? I have not cranked it since however did pull the oil measuring stick and it appears to be nothing there but oil. Please advise and recommend any corrective actions that you think I should undertake.  The model engine is: CV409S and is a Sears front engine.
Thanks, Jim  

Answer
 Hello Jim:

 This has Become a Common Problem. Install a Manual Cut Off Valve in the Gas Line and Turn Off the Gas Flow to the Carburetor when the Engine is Not in Use. The Gas Entered the Oil by Way of the Carburetor and Cylinder. The Carburetor Float Allowed Gas to Enter the Cylinder and Flood it. Then the Gas will Seep by the Rings and Enter the Crankcase. Rebuilding the Carburetor only Helps for a Few Months. That's Why I Suggest the Manual Cut Off Valve. Hope this Helps. I am here if you Require more Assistance. Thanks.

 Good Luck

 Respectfully

 John