This is a topic that usually results in lots of response... all have opinions(!)...
Get hold of a Briggs and Stratton one cylinder workshopmanual. Mine have part.no. 271313-12/00 but that is in Swedish.
Read the small section on oil/lubrication/service. It is the best presentation of an answer to your question that I have ever seen or heard
I like and prefer Omega Lubrication - special oils for heavy duty machinery. "pay through the nose" prices mind you...
Magna Industrial Co. Limited
click on Omega.
(and once again is my reply cut short as it is published... this is getting annoying!...)
The text I did write contained these data:
757 15W/40 for temps less than 35/40'C.
757 20W/50 for temps that goes higher than 35/40'C
690 80W/90 in the transmission.
Quaker State 20W/50 and 75W/90 mineral is an option to me.
...and that is a cheap fully synthetic oil... I do not like synthetic nor semi-synthetic oils... I planned to flush my R80 engine, still not gotten around to finnish the flushing...