Carhurga, I was surprised by the low amount of carbon on the valve stem (no picture but +0.1mm on the diameter under the heat) and in the combustion chamber. see picture.
I run with a 25:1 ratio with Stihl HP super oil and SP98 E5 gas (5% ethanol max, we have no more ethanol free gas in France).
On my side I am also doing some experiments, and that's why I use a 9kg Beaver. It is easier to land after a engine trouble as a 14kg warbird...
Of course no one is obliged to follow my trials, but it's interesting to share experiences.
JM