RE: What Can Be used insted of afterun oil?
Get some "Air Tool" oil...the stuff used to lubricate pneumatic tools...
It's designed to lubricate and prevent rust.
If you're using a fuel that has all synthetic as it's total oil content, and haven't experienced rust (yet ) then you either live in a very dry climate or are just lucky.
Some castor in the fuel's oil package helps prevent or lessen corrosion.
I have some airplane engines going back to the 1940's that were run on fuel w/ 20 to 25% castor oil fuel...and the bearings are in great shape. I have newer engines that have cheaper bearings (lower quality steel ) that have rusted seemingly over-night when run on fuels with no or very little castor and when I forgot to run the fuel out/use ARO.