When I add castor I figure in bumping the total oil up to about 20%, erring on the side of more oil.

128oz fuel at 18% total oil has 23.4oz oil. Adding 4oz castor to a gallon of fuel gives you 27.4oz oil, and a total oil content of 20.75% (85%synthetic, 14.6%castor).

Don't forget that adding oil also reduces nitro content, though not by a noticeable difference. If you have 10% nitro fuel, and add 4oz of oil, you end up with 9.55% nitro.

This is pretty convenient, at least for me, as Price Chopper sells 4oz bottles of castor oil for $2.75.

*note: someone above talked about running OS engines on 2x% oil; the manual does say MINIMUM oil content of 18%, so I agree that 2x% is pretty good.