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.
10w - 30
SAE-30
check you tube under removing the govenor on a Briggs and stratton
SAE 30 SF or better rating
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
Generally Briggs calls for 30W but you can also use 10w30 especially if you are using it in cool temperatures.
40oz
10w30 0r 10w40 both seem to work well.
Sae 30w if it is over 50 Fahrenheit. 10w-30 if it is under.
Click the link and find your engine for the exact capacity.
SAE 30
RJ19LM champion.