At Fleet Farm, ran across something called Tall Timber oil. It seems to be a local thing, and it's made in Minnesota. It's intended for chainsaws and other air-cooled small engines and it only comes in packs of 2.6oz bottles to mix with one gallon of gas. A single bottle is $0.99, so I figured I'd give it a try.
The Tall Timber seems to run well so far, and it actually seemed to run cooler when I put my hand on the bottom of the crankcase, though I can't be sure of that. It's rated International "ISO-L-EGD", so It should be adequate for these engines. I usually run Amsoil Interceptor, but it's intended for liquid-cooled snowmobile or outboard engines. Nobody near me carries Saber, which would be better suited to a moped engine. I may order some Saber to try, but for now I may keep using this Tall Timber stuff. Has anyone else had good luck with it (or even heard of it)?