That’s what I though too. But after examining 6 used pumps I can only see wear on the impeller and not on the end of the shaft. So I took a spare C head and it appeared the recessed area was not touched only the area where the impeller would hit was there wear. So???
Two years ago we took a trip to the west coast. By the time we arrived home it was a 8700 mile trip. Tightened a screw on the door handle and changed the oil that was it. Very fun time.
I fixed a model a engine with 10 psi in one cylinder and 0 in another. Still ran but no power! He had just gone one a one hundred tour with it. It was the valves.
I do run a filter on my engine and change the oil and filter at 3000 miles. I have 60000 miles on this engine. 15-40 Mobil oil. I am not saying this is correct, it's just what I do.