Small Engines (Lawn Mowers, etc.): gas in the oil, piston rings, head gasket

Question
Recently rented a house and a Lawn Machines lawn tractor was left for me.  The engine is a Tecomsuh OHV built around '99.  Finally got it running, but found gas in the oil. The oil is very thin. Could this mean piston rings or head gasket?  I did get it to run for 15min, but now starts and stops.

Answer
I would check the carburetor first. This is something I see every day, sediment accumulates in a carburetor bowl and causes the needle to stay open, allowing the fuel to flow through and fill up the crankase. Make sure after cleaning the carburetor you change the oil.