RE: Fuel recommendation for YS63FZ-S
Since I find Magnum I does all I want with my YS engines, that's what I now use. One of my buying pool normally burns Magnum I but likes to stock some Magnum II for competition purposes on hot summer days, to make up for the thinner air. I prefer the convenience of using one blend for the year, and my planes have adequate power without going to higher nitro. I can't say I ever noticed the difference in idle rpm or reliability between I & II, but did notice the better "gas mileage" and the easier pull on verticals. I would say again, the economy of II vs. I is probably about the same, so if I thought about it a bit I might persuade myself to run strictly II. If you've never tried II, you ought to get (or split) a case just to try it--or get a season's supply of II, pay a little more per case but burn less for a day's normal pattern flying and come out even at the end of the year.
I do remember a day when I was flying my Venus with a YS 53 (not 63), hauling it all over the sky on 30%, with an audience including a couple of old hands who were speculating as to what engine I had that was performing so well. My buddy told them it was a YS 53, at which they scoffed and told him "No way! You're full of ####!" When I showed them the engine, they thought about it a while before one thought to ask what fuel I was using. When I said 30%, their response was "thirty bucks a gallon--who can afford that!" But the bulk price from Magnum was pretty close to what they were paying at the LHS for 10-15% fuel.
Another thing about the II--I had a case of it, and only burned one gallon straight from the jug, to satisfy my curiosity. The rest I blended with lower nitro just to get better use out of other gallons of this and that. I never had a problem with blending different brands of fuel, although I've been careful not to blend castor-containing fuels into my YS engines. But I do like castor in my ringed two-strokes, and never had trouble with them either as far as mixing fuels pretty much indiscriminately. I've also blended CoolPower and Powermaster high nitro with lower nitro and FAI blends of other fuels (mostly S&W & Wildcat) with no complications performance-wise.