The best answer I can give is . . . . Maybe.

If you're starting out with a new Fox .35, you can just bring up the total oil to 29%, starting with Cool Power 10% or 15% Nitro. If you're gonna use the Cool Power (base) with an older, well-worn Fox, you're very likely to end up with a dead engine, as the synthetic oil in Cool Power will scrub away the Castor "seal" that the Fox has built up.

Of course, you could order a Ceramic or ABC Piston/Liner set and avoid that particular problem, too. And especially with the Ceramic set, you'll get a smoother-running engine, too.

Ralph

I think you will be fine kicking the oil % up to 29% with added castor.  I'm presently running a Fox I bought in 1977 and put a couple thousand flights on before putting it into its present airplane.  It has @ 150 flights on it with Sig Champion 10% nitro, 20% oil 50/50 synthetic/castor.  It has fine compression, gives first flip starts and reliable runs.  I probably flew it on all castor fuel in the good old days, as the head is baked on black. 

Thanks all....only one known control liner in my area (there apparently was a huge club here in the 70s), and he owns the hobbyshop!  I'll ask him this week if he could get some Sig castor into the shop and follow you folks' advice.  No lack of R/C out here in Nova Scotia as there are four clubs all within a 60 nautical mile range, but control line just doesn't seem to have a following.  Given the declining number of R/C fields, I think the time is ripe for getting grass roots c/l model flying off the ground again.

Okay, I know that this has been beaten to death, but.....

There you are, in a town with a nice little LHS that only sells Omega Cool Power 5-25%.  They have to cater to the R/C folk.  But you, having flown R/C and beginning to learn how to fly control line, want to return to the Fox .35s that powered your uncle's Nobler and P-39 in the 60s.  Can the Omega fuels, with castor additive (I've got a source), be used in a Fox  .35 Stunt?  I know that the Fox is a lapped, bushed engine, but what I've read on this forum leads me to doubt that Cool Power is a safe adventure.

Hello Kirk and welcome
you may want to read the fuel and engine article at the first of this forum, this is a small excerpt from it.

"So what percentage do you try? For motors like Fox .35s, OS Max 35s or the old McCoy’s and K&B’s, use a fuel with 26 to 28% oil content; preferably half castor and half synthetic, up to 75% castor  is OK. These  motors have very small bearing surfaces, and are subject to much wear and heat, most are all plain bushing motors and most have unbushed rods. They need a lot of  oil  to help cool the engines. Since these motors run hot, they need  extra oil to keep them lubed,clean, and to carry out heat . If you have one of these that is  in very good  shape but, is  just starting to get some brown or black varnish plating on it, the synthetic mix will clean it  up for you, resulting in increased life.  Do not use the synthetic  blend in an old motor that has a lot of time on it with all castor fuels; the synthetic will remove the castor varnish off the piston and sleeve and will in some cases, leave you with the worn-out motor that had to start with.  Also always try to NOT use  prop shaft extensions with these engine, as it adds a  lot of  wear on the crankshaft bearing.
So what percentage do you try? For motors like Fox .35s, OS Max 35s or the old McCoy’s and K&B’s, use a fuel with 26 to 28% oil content; preferably half castor and half synthetic, up to 75% castor  is OK. These  motors have very small bearing surfaces, and are subject to much wear and heat, most are all plain bushing motors and most have unbushed rods. They need a lot of  oil  to help cool the engines. Since these motors run hot, they need  extra oil to keep them lubed,clean, and to carry out heat . If you have one of these that is  in very good  shape but, is  just starting to get some brown or black varnish plating on it, the synthetic mix will clean it  up for you, resulting in increased life.  Do not use the synthetic  blend in an old motor that has a lot of time on it with all castor fuels; the synthetic will remove the castor varnish off the piston and sleeve and will in some cases, leave you with the worn-out motor that had to start with.  Also always try to NOT use  prop shaft extensions with these engine, as it adds a  lot of  wear on the crankshaft bearing."

You can with success, as Marvin stated add 15 to 16 ounces of castor to either Cool Power or Omega, But never run them as is in a Fox 35, many people have wiped out the rod on 1 flight doing this, by the way Morgan fuels are measured by weight, not volume so the 8% oil figure is really just a tad under 15% by volume, The last time I did a boil down test on their fuels I got 14.8 % oil.

This is the link http://stunthanger.com/smf/index.php?topic=4037.0

REgards
Randy