Everything I have seen, when labeled Helli fuel, its 100% synthetic oil. No caster. Thats the main difference. Car fuel will have anywhere from 10% for race up to about 18% oil for sport formula. Many car fuels will also have a more synthetic than caster content as well. But most of the percentages differ from manufacturer. Regular airplane fuel will have a caster/synthetic mix. There is some debate on the use of all synthetic oil brand fuel, because some syn oils will burn off at the temperatures our engines run. Caster is good up to 700 or so degrees, but I am not sure so don't quote me on the exact temperature. And most synthetics will burn up at 300-400 degrees. Older engines required the use of caster, but in new models, many use synthetic only. I do not personally know anyone that does. Someone that does can give you their opinion about it. Personally I like having the real stuff for peace of mind. I am not for sure but when you see most Helli engines they have that huge square head on them to dissipate heat a lot faster than a normal head will. I think that is why they can run the synthetic, just because their motor temp is lower and will not burn off the excess oil.