Hey, I just found out the hard way why after run oils should be used. I bought a plane off a fellow member, and it came with a used GMS 47. It ran great for about 3/4 gallon of fuel, then i started having problems. At first, it was making a weird sound at low throttle settings. Then I had a deadstick on the down line of a hammer head. (it died when i chopped the throttle) and today, it just quit. half way through the flight. I never had a lean run, and initially suspected dirt in the carb. when i went to turn the engine over, it was really tight. It always had great compression, but it was tight all around. I just tore it down, and found the bearing in the very front of the engine had failed. The crank shaft was really rusty and corroded.
My guess was that this engine had been sitting for a long time without any oil in it. The fuel i use says not to use after run oil, but i do anyways.
So for anyone new to the hobby, or anyone who doesnt use after run oil, this is what will happen.