RE: FAI fuel
100% castor oil lubrication is ideal for plain bearing engines. The thickness of the castor oil helps the crankshaft "float" inside of the brass bushing for smooth performance and long life.
What I don't know is, how well do commonly available plain bearing engines respond to 0% nitro?
The Thunder Tiger GP series, O.S. Max LA series, MECOA K&B Sportster series, and some currently available models from Enya (like the SS40 SNV or 09-IV, for example), are all examples of readily available plain bearing engines that would be ideal candidates for 100% castor lubrication.
My guess is that the Enyas or the K&B Sportsters would be most likely to run well at 0% Nitro and 20% castor. The Thunder Tiger GP series would probably be a little finicky with regard to tuning, but would probably run well if tuned correctly. I just don't know if O.S. Max LA series engines run at a high enough compression ratio to operate well at 0% nitro.
Hopefully someone who has run these engines at 0% nitro can comment.
The Thunder Tiger GP series would probably be ideal test subjects if you're looking for inexpensive glow operation. The GP-42 is available for $59.99 while the GP-61 sells for a mere $85.99. Both come with 3-year factory warranties and both are very good performers. If tuned to run well on FAI fuel (0% nitro and 20% all castor), these would be the ideal combination of inexpensive to buy, inexpensive to run, powerful, reliable, and readily available.