My Cub Cadet 1862 (now about 12 years old?) has a Kohler twin opposing cylinder engine, I run it at full throttle for mowing, and always have. The spark plug get changed about every other year, ditto with the oil. Needed a new battery once. Other than that, the engine has never been serviced. And there is NO "notch" to indicate where to run the throttle when mowing, but as I have a couple steep hills, the tractor needs full throttle to make it up the slopes.
I have been told years ago, that running at full throttle is better than running it lower because it reduces the chances of the engine bogging down when the grass gets heavy, etc. Again, don't know if its true or not, but was told that the engine will sustaing more potential damage when it bogs down than by running at full throttle. Can't say if there is any truth to any of that, but I know that my engine never has needed to be repaired. I do the same with my JD push mower and its Kawasaki motor, run it at full and that thing is 15+ years old, it gets a new spark plug about every 5 or 6 years??? Starts up and runs perfectly, and always at full throttle.