some of the best performing fuels I have used have always left a heavy black carbon buildup on the piston......I actually feel safe when I see that as i know the fuel has a good amount of caster oil...the fuels that leave no residue are the ones that scare me ! Now of course I am into screaming the jeebus out of my engines, so I tend to stay away from all synthetic mixtures.... to me black residue means caster oil...clean piston means synthetic oils....caster protects better at high RPM's so thats why I prefer it........... However seeing a black buildup with synthetic mixture means it needs some tuning LOL !