There's a big difference between the demands on oil between auto and lawnmower. The auto engine is water cooled and that water is air cooled by a massive radiator with fan. Therefore, the oil operates in a well defined range. Conversely, today's 4 stroke lawn mowers have neither water or forced air cooling. In addition, they are typically operated in the hottest ambient temps of the year. Tough job for any oil, and excellent reasons all for using synthetics only. My Honda mower specs 10W-30. I use M1 0W-40 primarily for its higher loading of anti-wear additives. I change oil sooner than specified.
Autos have proven to be extremely forgiving of oil experimentation (witness this website). Really, as long as there is oil on the crosshatches, your engine will survive.