ORIGINAL: Remby
Andrew;
The Klotz I speak of is pre-mixed, in the Sig 25% Champion fuel. I am unsure as to which type they use, but know using that brand causes the flashing with Cox engines.
Remby -
I believe the Klotz lubricant in the Champion fuel is the original Techniplate, which is all synthetic. They list their Champion fuel as 50/50 Klotz/Castor. Since they carry Techniplate as one of their oil additives, likely it's used as their synthetic additive. The flash points for Techniplate and BeNol (castor) are 460 and 555 degrees respectively, which could certainly explain your piston coloration.
I've been buying high nitro/low oil car fuels, then adding BeNol to bring the total oil up to about 21% (18% castor) - with the higher nitro content, I can add oil and still retain a nitro percentage around 22.5%.
andrew