Lawn mowers are simple beasts with splash lubrication. Back when Briggs and Stratton first started producing lawn mower engines in the 1940s or so, 30 weight oil was the norm. I have a few antique engines from the 1940s-1960s and many of them specify "SAE 30 in summer, SAE 20 below 32 degrees". I have a 1971 Gravely walk behind that actually says "Mobil SAE 30" on the serial tag.
If you open a Briggs flathead from 3 years ago and one from the 1940s, they aren't much different inside. SAE 30 is also pretty shear stable. More modern small engines like the OHV and OHC Hondas, OHV Kohlers with hydraulic lifters, etc, will specify 10w30 or 5w30 synthetic in the owners manual for improved efficiency.