There is a lot of "what ifs" in your post. I don't think anyone knows for sure. More oil changes would have been better, that is for sure, especially if the person was doing a lot of city driving.

My Benz's recommend changing oil every 5,000 miles (300) and 7,500 (420) , although I change it much more frequently than that.

My manual says that I should expect to consume 1 qt. oil for every 2,000 miles on both cars. 300 burns more and the 420 burns less.

You are burning a little more oil than normal. It would be expected that you would burn more than normal in the future, but additional carbon deposits could act as a seal.

Maybe you may want to add one qt of STP oil treatment to thicken the oil. This may reduce oil consumption around your rings, IMO. I never used in on a Benz, but used it successfully on American cars.

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'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)