At the risk of being a heretic I say it depends on the Fox in question

Old skool, well worn, I think 28~29% oil mostly or ALL castor is the recommended mix

I found two Nib Fox 35s and started them on a diet of 10% or 15% Nitro with only 25% oil...20% Castor and 5% synth

Both seem to like my mix both for bench run in and out flying.... not sure what plug came with each so bought RC longs...still running fine on the 15% and 25% oil with ( as delivered) unknown installed plug....no burp modification despite many recommendations for the bypass mod

BUT, I am a sedate sport flyer and not doing any burp causing sudden maneuvers

I doubt oil between 24% to 29% is THAT critical if the NVA is set right and no too lean a run is likely

Thanks. The engines are very clean ceramic and ABC versions that have always run with some synthetic, usually 50% of the oil.

Larry,
With the ceramic and ABC setup you can run them down at 25% total oil (also works OK for the original steel/iron). I have had slight crackling (pre ignition) with high oil % 27-29 (it raises the compression). Once I drop to 25% (50/50 castor.klotz) the crackling was gone and it had a very solid run. I would add 3 oz of Klotz to your 22% GMA.

If you are going to run them with a muffler don't load them with a 10x6 prop, use a 9x6 (open face the 10x6 is OK). Also, if you are using aluminum pads on the motor mounts make sure you put 1/32" plywood crush pads under the motor case lugs to allow the mount to be solid and allow for the lug casting relief angle.

Best,   DennisT