Keithed - please don't take this personally, but your response lacks analytical rigor and, unless you have more data to show, is not a basis for your conclusion.

First, it would be interesting to determine who makes the OE tensioner - which was the thrust of the original question. Why do you say that only the MB part should be used? How do you know whether MB even makes this part? Your data on this would make interesting reading.

Second, what is the logic for the mechanic's statement that only original factory tensioners should be used? Does he have dozens of data points that show higher failure rates with specific manufacturers' products? Is he conflicted because he can command a larger mark-up on the purportedly OE part?

Maybe you have more data you can share supporting your response? I would be interesting in learning more of your "...lots of research..." as I am about to replace the tensioner on my 1990 560SEL.