I am unaware of a 10w/40 offered by Rotella, but I'm a fan of using the diesel oils in my gas engines. Be it my mower, atv, motor scooter, the two motorcycles.....even the Camaro, gets a diesel grade oil.
You will not harm your mower by using it....my 1000 series Cub Cadet, is in it's 11th mowing season, has 1,049 hrs on the clock, and still runs great. Have used Mobil Delvac 15w/40 conventional since almost new. No make up oil needed between 50 hr changes, no blue smoke....just keeps purring along. The only advantage I see with the synthetics, would be in really cold weather with starting, and improved initial oil flow.....but I don't mow grass at 5 or 10*......now the Kioti Tractor is a different story, so Rotella T6, 5w/40 synthetic is a good choice.