I've used 10/30, 10/40, 20/50, sae 30 conventional in various mowers and ope over the years, with varying sump sizes. I think I put a quart of castrol 0/40 in a generator once because it was the last of some clearance oil I had. Right now there's castrol sae 30 in my push mower, 1/2 qt capacity; a mix of delo sae 30/formula shell 10/30 in my dad's riding mower, qt and a half capacity; mix of Fred's 10/40/havoline 5/20 in my grandmother's riding mower, qt and a half capacity. Never seen a reason to over think ope oil, and that mindset has finally trickled down into my automotive oil selection. Buy what's reasonably priced.