After run oil is not necessary unless you are going to store the engine for awhile. I would recommend after run oil if it is going to be 2 weeks to a month or longer before you run the engine again. I personally feel if you use fuel with castor oil you only need after run oil if you are going to be storing the engine for a long period of time. Over the winter, for example.

After run oil should be available from your local hobby shop (LHS), Marvel Mystery Oil and Marvel Air Tool Oil will all work just fine.

Put a few drops down the carburetor and rotate the prop several time to move the oil around. For long term storage put 5 or 6 drops in the carburetor and 2 or 3 drops into the glow plug hole. As before, rotate the prop to move the oil around and to thoroughly coat the bearings and the inside of the engine.

Regards,
doubledee