here in the UK we have Weston UK fuels, thankfully, because this stuff about castor being able to run hotter is now untrue.
the oils in Prosynth and Liquid gold run 30 degress hotter than castor can by which time castor is a brown sticky engine gumming crud, the weston stuff is a mix, one oil burns off taking the heat, the other doesnt and lightly covers the plane, as long as you see this covering you know the oils doing it's job.
my engines run cooler, faster and show no sings of wear and NO BROWN CRUD!
apparently prosynth can also be used on 4 strokes, no more carrying round two bottles.
Liquid gold has slightly more oil and might be a better first intro to synth fuels ( like they are hard
) Prosynth makes hardly any smoke, but i still run rich so i have perfect mixture for when i;m chucking about or hovering.
irvine sport is ok, it's used here, but as synths go its a messy one.
it's still possible to run lean and do damage, but it's not hard to tune an engine propperly, a tip do it nose high, that way you wont run lean in the air.