Straight 30 when possible, 10-30 OK too especially in cooler temps. That said, I took over my father-in-laws yard mowing after he passed away 10 years ago. He had an enormous amount of O'Reilly's 20-50 on hand that would go to waste if I didn't utilize it. It's too thick for the cars, so, I an using 2qt's/season in the mower. It LOVES it! Of course, I am mowing in 100 degree heat for 4+ hours every time. Still, it has worked great!