Run at least 32:1. I run 60:1 on synthetic and have been for a year with no problems. Had it all apart a while ago and no scoring or anything.
These aren't the best designed engines and will usually 4-stroke a little bit under light throttle, but at WOT under load it should run nice and clean. If you have 4-stroking you're running to rich. So move the e-clip up a notch.
for mine to run well though, I had to re-jet the carb smaller. I now have almost no 4-stroking. But I'm not running to lean. Just right.