Small Engines (Lawn Mowers, etc.): Oil in crankcase?, crankcase oil, briggs and stratton

Question
I have a Craftsman lawnmower with a 7HP Briggs and Stratton engine that was given to me by a friend. It sat unused at his house for about a year and a half. When I got it I cleaned out the old gas(was all gummy), cleaned the spark plug, and the carb, and fired it over. It smoked like crazy for about 20 minutes, and then would only smoke under heavy load. The smoke is always white. I went to use it today, and it smoked upon starting, ran for about 5 minutes, the shut down and would not start. I checked the oil, and it looks very gray, and smells heavily of gas. Originally I figured it just needed to re-seat the rings, but as much use as I have given it should have done that by now.  Any advise would be appreciated.

Answer
 Hello Walter:

 Change the Crankcase Oil. Allow the Crankcase to Vent to Air for 24 hrs and then Refill with Oil. **DO NOT OVERFILL**It is Contanimated with Gas. The Carburator Float Needle isnt Stopping the Gas Flow fto the Carburator when the Mower is Not in Use. This Floods the Cylinder and the Gas then Leaks by the Rings into the Crankcase. Install a Cutoff Valve in the Gas Line to the Carburator and Turn Off the Gas Flow Whenever you are Not Using the Mower. Check the Needle and Seat in the Carburator. Sometimes the Slight Leak Cant be Stopped by the Needle and Seat. This is the Reason for the Shutoff Valve. The Newer Engines have a Shutoff Soleniod in the Base of the Carburator. These are Also giving Problems to Owners. Hopefully there is No Damage Done.  Hope this Helps. I am here if you Require more Assistance. Let me know what Happens, Please. Thanks.

 Good Luck

 Respectfully

 John