ORIGINAL: JKinTX The castor content needs generally to be higher in hot weather than in cold, the nitro ratio can change also to suit. That said, back before synthetic oils, we ran the Cox fuel out of the can here locally and we had more blown (fouled) plugs here in the Summer when it got above a certain temp/humidity level than at any other time. Today, I would drop the castor off a tad and run slightly more synthetic.
Back in the late 1970's, I was running K&B, Duke's, and to a much lesser degree the Cox fuels, all with the higher nitro content (15% - 25%). They worked satisfactorily in the reed valve .049's. Since they were Castor based, after so many flights, I had to devarnish the cylinders. Didn't have a problem with plug fouling, but then I always peaked out the engines with the nose of the plane pointed 45% up.
Now that I am retired, I have acquired some old legacy baffle piston engines, Enya, OS Max, OK Cub, AC Gilbert, Fox, McCoy, K&B and etc. Also have a bunch of Coxes with the ball and socket con rods and aluminum crankcase plain bearings. Those are the ones I'm modifying the oil content of the modern fuels up to standards, mix for small ones, another for larger.