Poll ! Poll ! Poll !
I'd have to vote "all of the above" cause I bought fuel with a specifically low synthetic content that I add a small amount of "Benoil" pure castor to . Between the castor percentage in my fuel and a proper after run flushing before putting a plane away after a day at the field , I haven't yet rusted a set of bearings in any engine I've bought brand new . And come to think of it , just about any used engine I've bought has always had at least a bit of rust on it's bearings . I guess my rituals work .