STC AvGas

www.eaa.org/eaa/pilots/eaa-stc-program/a...proved-engine-models

Available from EAA too 

It looks like a C-85 STC for Mogas is only about $150 as well via mail (Petersen Aviation Inc).

92 Octane non ethanol will work fine in your C85, but you’re suppose to have the STC to do that.

I have Ethanol free gas right next to my flying field.  If I check for Ethanol each time I get gas, am I safe to use 92 Octane in my C-85?

The STC is for Mogas. Not AvGas. We don’t need an STC to burn Aviation gas.
I have the Mogas STC. It’s not readily available around me so I don’t burn it often. I have a C85 Stroker. My AP who built the engine, said not to put anything less than 91 Octane in it. I would have to drive about 50 miles round trip to get it but for about $2.29 a gallon. It might be worth if. Just not convenient to do it very often.

I use food coloring, small bottle of McCormick red food coloring that allows one to put just one drop in a gas sample collected in one of those clear plastic quick drain tools directly from the 5/6 jug before pouring in to the plane. If no alcohol, the small one drop " ball " stays intact as it falls to the bottom - any alcohol causes the "ball" to break up - easy to see.
Using auto gas will result in a 'blacken" exhaust more so and one may think that the engine is running too rich but the condition is "normal", at least for me but the auto gas sure helps with the plugs as indicated. I do run 100LL at times