10W30 is fine. Been running it in Tecumseh and Briggs-engined Craftsman mowers for 20 or so years or as long as 10W30 has been common w/o problems.
I can't remember when our autos started using 10W30 - 1982 or so? Pretty sure the Pinto took 10W40, the Pontiac J2000 was for sure 10W30 - it was my first car. That car (named Thunder as it would backfire as the computer was half-dead!) taught me how to fix cars. A blessing in disguise. My 2.5 year old, on my lap now, is getting a junker as a first car to learn the same lessons.
Once the cars started using 10W30, that's all we bought.
Up north, we were cutting over an acre with 3 HP self-propelled mowers. Took all Saturday, every week. It sucked. We'd change oil every 20 or 25 hours and it would get real dark. This was very hard service - we wore several sets of wheels off of them before they wore out. And wear out they did - about every six years - they would gradually lose power and smoke bad. Have fond memories of going to Sears with Dad to buy wheels and cogs...
later,
b