My homebrew 5% nitro stunt mix winds up at around 28% castor, but I do toss a few ounces of Klotz in bringing the total oil content in the 30% range. Just a few ounces of synthetic per gallon seems to keep the varnish down. My 50 year old Stunt .29 was pretty badly varnished acting erratically. After an hour or so of running with the bit of Klotz mix its piston looks cleaner and its behaving again.

Basically, I'm cloning Fox Super Fuel and adding a few ounces of Klotz on top of that.