I use Power master 10% which has an 18% blend of castor and synthetic. I add an additional 4ozs of castor to each new gallon. My planes are a little messy at the end of the day, but because I don't run my engines leaned out to peak rpms I don't have castor varnish issues. More importantly, if something happens to a fuel system in flight and an engine does go lean the odds are it won't be damaged (if I land right away).

David