I'd get some Lubro Moly 235.0 spec 85w90 gear oil, that should be what was originally spec'd in that car. It's non-synthetic but it is what's approved by MBZ for use in your diff. I used it in both my Benzes, including my W208 CLK430. I found it locally pretty reasonably-priced. I'm just a stickler for approved MB fluids.

Once you get the leak fixed, drain and fill with that same gear oil and motor happily.

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1995 E300 Diesel
~300k