I'm trying to figure this out, right now, with my fleet. I've got my son's garden tractor with a Kohler Pro 25 hp twin cylinder, CV730 type. It's almost running, new parts arrived, got to get them on. The tractor's owners manual (Pict_1) and the Kohler service manual (Pict_2) both recommend 10w-30 for above freezing temps, and 5w-30 for BELOW 32 degrees only. These are older manuals with older school of thought, and I DO NOT believe that now. Seems contrary that 5w-30 is disallowed but 10w-30 is just fine. Heck, we're talking about a completely different question--40 weights vs. 30 weights. The tractor has been parked for awhile and I need it to move, and it needs an oil change.
The 3rd pict is from my John Deere Ztrac with a briggs stratton twin. It is a newer zero turn with a newer manual, and what I believe is the CURRENT recommendations for air cooled engines. It shows all the 30 weights (straight 30w and the multi-grade 30's) equally good up to 104 degrees, but all the 40 weights (again, straights same as multi's) good to 122 degrees. The 40 weights are the best per the Deere folks, but 30 weights work too. It also says straight weights don't get past the rings (less oil consumption) as much as multi-grades (worse oil consumption). I put in new oil every year, and that's now.
I've a buttload of 5w-30 to use up--from a Walmart clearance buy. Per the John Deere folks, all's good until I mow in 104 degree weather; this is not a problem as it won't happen. I've used it in that zero turn in it's early years, then I started using 15w-40 diesel oil. Which is almost all gone now. Since the Kohler engine is pretty much the same air cooled type, I think it will be just fine, and most of that cheap 5w-30 will go into my old Chevy 2.4l Equinox. But my next buy will be in15w-40 diesel oil.
All that said, I was looking for straight 30 weight while shopping for tractor parts. It is almost unobtanium. Not at Walmart--out of stock. I searched an Atwoods, a Tractor Supply, an Oreilly and Auto Zone, and a local Orschleins. Found at 2 places; 4 were out. If I wanted the best oil for Florida-Louisiana summertime use, it would be straight 40 weight.
Reminds me of my old VW days. You could get straight 40 weight back then, but in my recent search I found none. You air cooled VW guys out there, where you get 40 weight these days?