SAE 30 weight in warm weather or SAE 10w30 in cool weather. I highly recommend you use 100% Synthetic oil in any air cooled engine.
Briggs and stratton engines use different oils depending on the temperature to which the engines will be used. Generally sae30 oil should be used for mower engines
sae30
10w - 30
straight 30 or a 10w30
SAE-30
Briggs Stratton makes many different lawn mower parts. On their website one can find standby and portable generators, pressure washers, cleaning systems and snow blowers.
SAE 30 SF or better rating
The recommended oil is 5w30 full synthetic oil.
Generally Briggs calls for 30W but you can also use 10w30 especially if you are using it in cool temperatures.
Briggs and Stratton only recommend using their own brand of lawnmower oil. This oil is available in stores for around $6 for 18oz. This particular oil is designed to be compatible with air cooled engines.
SAE 30 weight full synthetic oil.
10w30 0r 10w40 both seem to work well.