Early on when I started flying, I used to use auto transmission oil for after run. But in the last 15 or so years I've been building and flying scale to the point that the engine is fully cowled in and access to the engine fuel line is not feasable. So needless to say no after run in these engines. I needed to pull the head, valve cover, and the push rod tubes to fit a new cowl around the engine head, and I was surprised that there was no evidence of rust anywhere. That prompted me to pull the rear cover off and found it to be free of rust also. Just oily residue. I guess I'm just lucky, but if you're able deffinately use the A/R oil for preventive measures.