As break in fuel, I use 5% nitro, 18-20%, 50/50 castor/synthetic, plus 4 oz. extra castor to the gallon. Got this from Clarance Lee in RCM (although I see he now does it with 10% nitro). If it's hot out use 5%. 10% could be more likely to preignite in hot weather which I believe could be quite damaging during break in. Just my thoughts.