My engine guy stopped by yesterday and caught me as I was about to fill my newly rebuilt onan with Mobile 1 synthetic. He said that I should be using a high zinc oil in my mower engines and specially for the break in period. I stopped right away and got some VR1 racing oil from the local parts store.
I will most likely make a permanent switch to vr1 for my small engines, but it is hard to stray away from what has worked for me for many many years.
Valvoline makes two racing oils, one with detergent and one non-detergent. If the container is marked "not legal for street use" its non-detergent. Most of the on the shelf valvoline VR-1 contains adequate zinc and detergent for passenger cars.