ORIGINAL: daven
The 80/20 mix that Jaka is talking about is used for FAI racing (not real big in the states) but bigger in Europe and Australia. The reason for 20% castor is due to the high expense of Nitro overseas.
The expense of nitro has nothing to do with it. FAI 80/20 fuel is specified only for certain international classes where ultimate power is the deciding factor (CL 2.5cc speed, Pylon and I think one class of free flight). It's purpose is to level the playing field so exotic fuels can't be used. AMA rules are only the local American rules whereas FAI are the international rules used by every other country.