Hi Buckets,
From research SG is not only acceptable but better for these types of engines as they have higher values of anti-wear additives, such as phosphorus and zinc.
My understanding is
one of the reasons oil manufactures change oil components is to keep up with anti-pollution changes such as catalytic converters.
During the combustion process phosphorus (and/or zinc) going through a cat convertor produces a poison.
So latter API specifications S-J,L,M,N shouldn't be regarded as better oils in this application, just different.
I also believe multigrades, with the added polymers which break down into sludge and not as good as singe viscosity oils in our application.
Though a car/truck needs multigrade I believe keeping the spread as close as your climatic conditions allow, i.e. use 20W-40 rather than 15W-50, but that's a whole different discussion.
Penrite SAE 30 and B&S SAE 30 and their equivalents are the best for small engines IMHO.
BTW also buying 5lt Penrite 30 at supers#!+ while on special
