I agree its just normal wear, which is part of the reason you try to avoid rotating the stems, as it disturbs it's "natural" pattern and can possibly cause undesirable results. Some manufacturers (Chevy and Ford) that I know of, actually addressed this problem by incorporating valve rotators under their springs, with a lobe that struck the valve lifter in an offset area, so as to make the valve spin as it actuated each time, theoretically causing it to not create a one spot wear pattern. MB didn't do that with these engines, causing that exact situation, which isn't really a problem, but could cause a shorter lifespan between valve jobs.

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Chris
64 190D R.I.P.
80 240D W/617 engine -for sale
82 240D -for sale