This from the OS Max Engine Manual for the LA 40, 46 and 65 engines:

Running in & ordinary use: 20% castor oil, 5% nitromethane
More power &flexibility: 20% castor oil, 15% nitromethane

Only readily available commercial fuel with 20% castor is Sig Champion all castor. I'm sure their regular Champion fuel would work OK. It has a 50/50 castor/synthetic blend but same overall oil content.