I personally prefer 25% nitro because it's a lot easier on plugs. Glowplug Boy offers terrific fuel now made by Fitz. The original owner of Glowplug Boy passed away and Fitz took over for him. Excellent fuel which offers different nitro levels . His fuels contain a touch more castor. If you've been out of touch with the cost of 1/2A fuel, your going to be a bit sticker shocked as not only is the fuel expensive, so is the shipping. Ritch's brew also offers terrific fuel for 1/2A. Don't e-mail him, call him direct. Brodak also offers 1/2A fuel which I've used with good success. You also have Stan from S&W fuels which will make any fuel you ask of him.
Thanks, Ken. There used to be long time hobby shops near me - 95 miles away either Amarillo or Lubbock, Texas. But they closed up a while back. After I was going to another in Amarillo, don't know if they survived Covid. Still got 4 gallon jugs of fuel, mostly a 15% nitro R/C fuel with a pint of Castor (Benol Racing Castor) added to give me 25% oil with 10% as Castor.
I recall that Peter Chinn, the UK engine guru recommended no more than 25% oil for the Cox reed engines, because their fuel passageways could not satisfactorily handle the viscosity of higher percentage oil (like the famed 29% preferred by some - especially for their Fox engines).
One stating 30% oil satisfactory use, perhaps used a lighter grade of synthetic oil?
I know my fuel mix, mostly for the legacy say, .35's - K&B Stallions, Testor McCoy .19 and .35 Red Heads, OS's Enyas work fine along with the Cox reedies. Yet my Norvel .061 Big Mig didn't seem happy with it.
Regarding ball and socket, I was flying with 25% nitro 20% oil (100% Castor) SIG Champion fuel back in the day, my Golden Bees, Babe Bees, .020 Pee Wees and R/C Bee were all happy on it, never had a problem. However, I was resetting the piston-con rod joint periodically on the .049's, when de-varnishing the cylinders coated with Castor residue. (Didn't have one for my Pee Wees. Now I do.)
Anyway, I hope the OP finds the fuel needed to make his Cox engines happy.