I use 25% all castor in my ST G51 because it doesn't give the kind of run I need with less oil. The Fox 35 needs the 29% oil and other cast iron piston engines will be happy with that too even though they don't really need it. There's sure to be a maximum amount of oil that's useable in an engine although I've never experimented to find out what it is but I'd guess maybe up in the 40% area.