When I was still using my iron piston Foxes regularly, I found that repeatable needle settings were just not readily available with less than 21-22 % total oil, by volume, at least not for a classic 4-2-4 stunt run. For Foxberg, I could get repeatability with a straight two cycle, but the usual 20% oil fuels for that event tended to allow the engines' piston to cylinder seal to wear out rapidly.
Meanwhile, Powermaster GMA 22%/ 10% half synthetic, half castor has been the base fuel I add castor to for the rare occasions when I do use an oldie iron motor. Adding four ounces of castor gives you a total oil content of 25%.