My engine was new last year and ran it full trottle all the time,changed oil at 8 hrs and continue same routine. To my knowlege of lawn mower engines proper cooling and just as important lubercation is achieved by running full thottle . Also lastly mowing quality is affected by a slower motor. I catch my wife with the throtle 3/4 open and show her to run it open.

Short answer: yes. As you slow the engine down the blades slow down.

Yes. Max RPM's does make a difference. Max hp. Max cooling. Max blade speed. Best mowing. I noticed last week my rpm's are down to 3480. I need to make an adjustment to 3600. I see a BIG difference at 3600 rpm.

IMO also ground speed is factor, the slower you go the better the cut, especially in tall grass, I never reduce throttle speed when mowing, but I will reduce ground speed. So going fast with limited throttle produced very poor cut reguardless of what brand mower you got. Throttle up and slow down!

Engineers design mower decks to spin at an ideal blade tip speed. The blades are designed to not only cut the grass but also to create an updraft of air to suck the blades into a vertical position so they can be sheared off at an even height by the blades.

The ideal blade tip speed is based upon the engine running at the governed speed of 3600 rpm. Now if you don't run the engine that fast, then the blades can't generate the same amount of lift as they were designed for. The cut of the lawn can suffer as a result. Now if you like to run your engine slower and you don't really notice a difference in the cut, then it doesn't really matter all that much.

Different species of grass and different cutting conditions can also affect the overall look of the lawn. If you're happy, then that's all that matters.

I kinda disagree with running engine at slower speed (reduced or slower speed is subjective 3000RPM. 2000RPMs yet 3000 my not matter, but I wouldn't want to go lower). too many factors to create failure. especially if an air cooled engine head cooling fins are not kept clean. liquid cooled I agree probably doesn't matter.

I called Kohler today about my 30 hp Command Pro. I'm getting close to the first 50 hp oil and filter change. I had a few questions about oil and filter, gas additives, and engine rpm's. The Kohler engineer said Kohler ships the 30 hp Command Pro set at 3750 rpm, no load. Well, I need to make an adjustment.

I doubt 80 vs 100% will affect much - as others have said, travel speed is much more important.

That said, I doubt very much engine speed was related to your failure. I expect there was a defect in the key or the slot.

It is also likely that there was a very subtle noise or vibration that had you notice might have only required tightening the set screw(s).

Best of luck