A new tensioner is less than $45 from FastLane and I believe it comes with new seals -- aluminum crush seals as ILUVMILS stated. I don't know if you just pop it in or if you have to prime it in SAE 10W oil as it says in the manual.

I don't see the seals listed in FastLane so give Phil a call.

Sixto
91 300SE
87 300SDL
83 300SD