Just some random thoughts...
My guess is that OEM oil is just rebranded famous brand oil, probably HDEO from the zinc content reported in VOAs.
If you want to look at oil specs and try to pick oil for, let's say a 20 something HP garden tractor with a filter, the the aforementioned M1HM in 1w30 or 10w40 is an awfully hard oil to beat. I was once told, back in 1994, by a Mobil engineer, that if I used My in my JD 318 I'd never have to worry about the engine failing. Well fast forward 22 years and 900 hours later an she runs like new, the only part in the motor replaces were two valve seals.
Small engines, filtered or not are rugged and survive in spite of abusive care, (or lack thereof).
Standard HDEO oils like Rotella, or Valvoline Blue seem to me to be an excellent compromise between all out spending and going cheap, while providing excellent protection. You can spend more, but you're reaching the point of diminishing returns.
The thing to remember is, what is "enough" protection? Once you have enough, anything more is a waste. I guess deciding what is enough is the hard part.