1QwkSport2.5r

Lou - don't you think 25% oil would be more appropriate for a smaller engine - maybe the .15-.20 class? A .40 lapped or not should be just fine on 20% castor oil. I looked at the paperwork for my Mark IV Fox .40 (one of the first compact .40s) and it states Gold Star (0% nitro - other web links show the Gold Star fuel to be 5% nitro...). Dukes Fuel (10% nitro), or Ten Plus (10% nitro) were suitable. I don't recall off hands at the oil content of those fuels are - I can't find the paperwork that stated it. The paperwork did not list a minimum oil content for the engine. The only paperwork that listed a minimum oil content was for the B-case .45/.50 at 17% oil minimum. My Mark V .40 has the head button with the tiny bowl and mega huge flat squish band. It ran on 5% nitro just fine as did my ABC .40 did with the same button. If one can find the cone shaped head button, then you have a lot more latitude for nitro as that head is suitable for 10-15% nitro. I've run 20% nitro without issue on the test bench, but may be too much once in the air.

For the heck of it, I attached a price list from 2008 for Fox fuels. Really just for no other purpose than to just post published stuff from Fox since there isn't much in the way of documentation from Fox on the web it seems. And FWIW, the Mark VI .36/.40 engines have the same fuel recommendations.