Can I use 10% nitro 22% oil (50% castor 50% synthetic)?

Absolutely unless it has been run a great deal entirely on castor.  Then I'd stick with castor.  When that happens a 'film' of baked on castor contributes to the seal between the piston and sleeve as well the bronze shaft bushing.  When this gets washed away by the synthetic oil the engine would be toast.  However if the piston and sleeve still look pretty clean and not burned you should have no problem.  These aren't quite as delicate in that respect as the .35 stunt.  Nitro is no problem.  In-the-day some of us who flew combat were using up to 30+% nitro for HP at contests.  The only thing we hurt were our wallets. Dukes Fuel and Missile Mist were no more than 20-22% oil.  Dukes was 10% nitro and Missile Mist was 25% nitro ( and purple dye).

Dave

Chris,
If you read Randy Smiths "Care and Feeding" article that is pinned to the top of this forum you will see that for the iron engines 25% - 50/50 is his recommendation. I have used this in several older Fox engines and it works fine. The 22% works for ABC/AAC/ABN ball bearing engines. I have used this in engines that were always run on castor and it is fine, never really gets all the castor out of the parts.

Best,    DennisT

Chris,
Here is an engine review of the Fox 36X BB. If yours looks like this, it has a single main ball bearing. I would still run the 25% total oil fuel as it has the iron piston/cylinder setup.

http://sceptreflight.com/Model%20Engine%20Tests/Fox%2036X%20BB.html

Best,   DennisT