Okay... I'm totally confused now.
My buddy just picked up a YS 1.10 for his new bird, which prompted me to pull my Magnum .91 out and dust off an old P47 ARF that I've had for a couple of years.
I always thought that all YS engines used YS20/20, other 2-strokes would use standard Powermaster 5-15%, and other 4-strokes would use Powermaster 4-cycle fuel.
Not seeing Powermaster 4-cycle anymore, and one of the shop guys said to just run the 15% that I've been running in my 2-strokes.

Now my buddy is telling me that he's read (on RCU) that castor will gum up the valves, and ALL 4 cycle engines are best run with 20/20.

Tower's write-up on the Magnum says:
2-Stroke Fuel (Recommended: Power Master 10%, Not Carried)
Break-In: No More than 15% Nitro-Methane, No Less 20% Castor Oil.
After Break-In: No Less Than 16% Castor Oil Should Be Used.
Synthetic Oil Content Should Not Be Used.

The instructions that came with the engine say:
The Magnum XL 80 & 91 Ringed Four Stroke Cycle Engines run on standard
commercially available model glow plug engine fuel. Because these
engines are Ringed, it is especially important to fuel with castor oil, with
a recommended 18%. Additionally, a fuel containing 5-15%
nitro-methane is recommended.

Who's right? Has the experience of the RCU members discovered something over time not indicated in the manual and on Tower's site, or is my buddy listening to the wrong person?