I guess that this depends upon your definition of "best." I run M1 10W-30 high mileage in my 22 HP Briggs for its 3.5 HTHS viscosity. Somehow I doubt that you will need to run a mower in temperatures where a good 10W-30 won't suffice. Briggs does recommend synthetic 15W-50 for commercial applications, should you wish to thicken things up a bit. Personally I wouldn't worry about it unless you always mow for more than an hour at a time.