Man!! That's a lot of nitro for a sintered iron piston and the rods of that period!!  When our club set up the Fox Speed event, the standard 5% Superfuel with the 29% all castor oil content was specified just to keep things simple and most guys operating them have that or similar fuel, and most clubs provide the fuel and a way to level the playing field. I think the guys putting on the Fox team racing events do similarly.  I have run as much as 15% nitro in my Fox .35s depending on conditions and altitude. And you are not the only one that has left the Fox fuel jug at home and then used what ever fuel you had on hand to get a couple of flights on an OTS model. If the the engine has significant time accumulated on it and is well broken in, I do not believe that you significantly hurt anything in the engine. I have done it and still have to use several paper towels to wipe to castor oil off the airplane afterwards. Just don't get below 20% total oil.
   Type at you later,
    Dan McEntee

Spec fuel for foxberg racing (with a Fox 35) has always been 10% nitro 20% castor 70% alky in my neck of the woods.

For my Fox 35 free flight builds I would use 25 nitro, 25 castor, 50 alky but they only run for a short time during the climb out.

Motorman

1958 NATS Stunt winner Bob Randall admitted years later to running 25% nitro in his Fox .35 Stunt (Gobbleswantz). He also admitted to having a bag full of Fox .35 Stunt run-in and ready to swap in when needed. I still have a hard time believing that he used a 10-6 Top Flite NYLON prop, as was advertised. What a crappy propeller!  Steve

     They weren't that bad when new. Otherwise, unlike now, people at the time were prone to keeping secrets, and running large amounts of nitro was one of those secrets. That naturally also requires you to carry around multiple engines, since they couldn't take 20-25-30% nitro for long.

      Brett