Would any of the motul 10w40 motorcycle oils be a good choice for a commercial mower in Georgia summer heat?
You bet! Any of the full synthetic oils with high ZDDP will work very well for OPE.
I use Mobil 1, 5W-40 TDT in my OPE, as it's a very robust oil with plenty of ZDDP. I use M1, 15W-50 in my generators.