The B&S manual that came with my Toro lawn mower with a never-needs-changing B&S says essentially "use whatever"...
I'm paraphrasing here, but it has a chart showing everything from straight 30 through 15w-50 depending on temperature. It does say that 5w30 is best for all-around use.
Confusingly, the Toro manual says to use either straight 30 or 10w30. Capacity is 15 oz, but there's an unexplained asterisk on that...