If you're using fuel with (even a small) percentage of castor oil and you fly regularly (say once a fortnight) then you probably won't need after-run oil at all.

Back in 1980, I took an engine out of one of my planes and threw it in a cupboard (without any after-run oil or special preparation). 25 years later (last year) I pulled that engine out and stripped it down -- not a spec of rust!

I had been using fuel with castor oil in it (and believe it or not, that was a 4-stroke engine!).

My 2-strokes (mainly SuperTigre and ThunderTigre) never see any after-run oil and despite being stored in an iron hangar over winter (without any insulation or heating), they've never had a problem with rust or bearings. I did replace the bearings in my oldest TT engine a few months back -- but that was only after it had done more than 200 hours and the bearings were worn, not rusted.

If you use fuel with 100% synthetic oil then all bets are off :-)