Yup, sounds good to me. 20% is a safe bet for oil content. Steel on steel likes castor. ABC/ABN plain bearing likes castor but works fine with synthetic. ABC/ABN ball bearing don't need castor but a blend is a safe bet. It should be noted that little engines (of modern design) follow the same rules. All relating to planes of course. Heli's and cars and boats have their own rules...
Again, it's not typically an issue for the everyday sport flyer. Whatever is on the shelf at the hobby shop will do (typically a castor/synth blend) so long as you tune properly. I used to run everything from .049 up to .65 on 15% nitro with a castor/synthetic mix. But an old (or old design) engine is definitely safe with 20% castor oil in the mix.
Good info guys. See you all in a few months when this comes up again.