I've been mixing my own fuel for over a year and run 5% nitro in all of my aero engines. My S90k is the picky one, it barely tolerates 5%. I lose 800rpm on FAI fuel (80/20 with all castor) but the exhaust note is "snappier" on FAI fuel.

I know it's overkill to use 20% castor in a ringed engine but it keeps them free of rust. I tried a 50/50 blend of castor and klotz and a teeny little bit of rust formed on the top of the liner. A little ultra fine steel wool took care of the rust and back to 20% castor I went. No more rust. The engine still runs fantastic.