Did the oil burning stop after the valve guide seals were replaced and then start back after the 3K miles. If so, then you need to pull the spark plugs (keep them in order) and see if one or 2 have more buildup on them than the others. If one is bad but the others are clean, then you have found the problem cylinder. It could be that one of the valve guide seals has slipped off. I have had that problem before. It could be that a valve guide has come out of the head. Had that problem also. Both of these problems can be fixed without major dis assembly. If all the plugs look about the same then you need to remove the valve covers and look at the cam oilers and make sure none of the plastic pieces is broken. That can also cause a lot of oil usage. I have had that problem also.
My 84 500SEL with somewhere over 260K miles uses about 1 qt/1500 miles at interstate speeds but uses almost none in city driving. I know the guides need to be replaced but I can not justify a job that big at this time just to fix 1 qt/1500 miles only when we go on a trip.

Paul

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84 500 SEL (307,xxx miles)