We see the black castor build up on pistons and cylinder heads pretty quickly here in the states also (racing engines: Nelsons and Jetts). We run primarily 15% nitro Powermaster (Castor/Synthetic mix). I like to clean up the piston top and head with never dull after about 20 2 1/2 minute runs.

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. Those engines are also running closer to 30k and build up castor varnish pretty quickly.