It's gotta be the castor. Get behind a plane in the pits using all synthetic oil and you won't smell that aroma. To get an idea of nitro odor, go to a dirt bike race. Smells a lot like 100+ octane race gas.
Oh, as far as castor goes, this might not be much of a problem with the amounts we inhale, but WWI pilots inhaled a lot of it as their lubricants were mostly castor based and they were in a constant mist of it behind those rotaries. Their first thought after landing was NOT "let's go to the "O" club for a drink", but rather "where the hell is the outhouse?"
