All of these suggestions are great BUT these engines, wether it be the B&S or Kohler are great motors. You will have your bad one by both brands. I am very easy on my equipment, warm up / cool down periods, etc.
I have a Snapper pucs mower with a 5 hp B&S bought in 78 (my Father), a Sears rider with a Tecumseh 10 hp bought in 69 (my Great Grand Mother) and a Troy-built tiller with a 7hp Kohler bought in 79 (my Father). I received all of these over the past year. NONE of them had decent oil in them when I got em.
Growing up I remember the Snapper getting oil changes once a decade MAYBE. The Troy-Built got regular oil changes, once maybe every 5 years. My Father said he never knew of the Sears rider ever getting an oil change and he cut his Grandmothers 2 to 3 acres every summer.
All of these motors have never been touched, though I did rebuild the Sears Rider's carb. They run GREAT. Now I know the motors of yeaster year are much better than the motors being put out today. Out of the 3 motors the B&S is the only one not cast iron.
If you change the oil regularly and not abuse these things they should run longer than the equipment attached to them. All of the cool downs while riding to the barn or while blowing it off and the warm ups while picking up the toys out of the yard will help but just use common sense. Don't shut em down from wideopen, dont't start and start em every 5 minutes, let it idle while picking up the sticks, and it should last for you to get your moneys worth and then some.
I am sure my Great Grand Mother, nor my father, thought about being able to leave the Sears 10XL to a 5th generation but my 12 year old son is able to cut grass with his Great Great Grand Mother's Sears 10 XL 35 years later.
Can you tell I am proud of the equipment I have inherited?
Now if I could just put my hands on my Grand Father's 1963 Massey 35 Deluxe my Great Grand Father bought new. It to has had probably 5 oil changes since new.