RG got it right. Alcohol left in the crankcase and on the bearings draws moisture. Here in Florida, rust starts forming on the way home from the field. Air tool oil / 3 in 1 / or any similar is fine for after run. The plugs might have carbon buildup, the engine might not be running cool enough, or they might have been run a bit lean once or twice. All killers. Use an OS 3 or 7 or 8. I've had some last a whole season here, and that's 11 months!