I wish I had a dollar for every oil or oil/gas thread on these websites!! Anyway, since I can't look away from a good trainwreck, I'll chime in. I use Redline, 40:1.
I had a plane with a DA170 that I flew for a couple of years before it bit the dust. When it did, it popped one of the heads off of the motor. When I tore it down the rest of the way, I was very surprised to see that there was no carbon buildup on the piston tops. None. There was just a little on the top of the cylinder, around the spark plug, but you could scrape it off with your fingernail. You could easily see the arrow on the piston tops, pointing towards the exhaust ports. Inside the motor was clean as a whistle, but I would expect that in any motor, since there's no combustion below the piston.