Who knows today???

I would expect a Dealer to be well aware of problems like that, but some just deny or say they never had any problems ...Don't know why, but they do.

There was actually a classs action suit on those b/c so many needed rings, but the outcome was the owners were not using Factory rec. oil..
That was the first thing I noticed on you post, so you seem to have taken proper and correct care...
Your preference should be with someone who knows Benz,,, as far as a leak-down test , any decent mechanic knows how to do that....
Again , that is the FIRST test I would want to see before making any diagnosis...
Even if you just go to a mechanic for the test alone and the results...that will get you on your way...I would not be as foolish as to make a low compression diagnosis w/o more info than a PSI reading and a guess.

Do a search on this Forum and you will see all the related threads on that engine with your exact condition. And then when you talk to a guy about checking it, you will know if he is not aware or hip, and can then decide if you want to use him.
Just ask if they can do a leak down test...if the guys says "A What? "... turn and run... Any good guy is simply going to say "Sure." .Then you explain your condition and the cylinder you seem to be having a problem with low compression and oil evidence... and that is all he will need to give you a diagnosis that we can not.

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A Dalton