If you want to use mystery oil, get the air tool oil, it has rust inhibitors that regular marvel mystery oil does not. Air tool oil alone works well. No real reason to mix it with ATF. Ask 50 guys what they use and you'll likely get 50 different answers on what to use. The best idea (IMHO) is to run the engine completely dry at the end of the day unless you're going to fly again in the next few days.

Drain the fuel from the tank, place the engine at normal start throttle and try to start the engine. Continue to do this untill the engine refuses to even pop. NOW add a bit of after run oil and spin the engine over by hand.

Long storage is a bit more oil and plug the muffler outlet and intake with a paper towel or plastic. The idea is to keep humidity out of the engine.