Briggs & Stratton 4t race oil by Amsoil or B&S straight 30w, depending on the engine.
My work is an Amsoil distributor so I get a nice discount on it.
Strigoi, years ago, they held swap meets in O.C. at a drive in theater. Forget the name now, but one time I saw this dude with the Amsoil display with the moving parts, and the friction thing- and I bought a few quarts. At the time, I had a Kawasaki KZ650, but it was a little bit hotter with a Yoshimura 717 kit in it, I thought I was King of the Ortega highway. (Route 74 going from the I5 to Lake Elsinore)
Anyway,
I put that Amsoil in my bike, and when I started it, I had to turn the RPM down. The oil reduced friction to the point where it was 500 RPM higher. I was pretty impressed with the stuff. I put some in my Toyota Land Cruiser and didn't see a difference like that, but for smaller engines, I imagine the benefits are readily available.
No, I don't sell it and I don't know anyone other than (now) Strigoi who sells it, just sharing an experience I had once long ago.