lsmalley, check your Owner's Manual for the type gear oil specified for the differential (normally 90W or 85W90). If your car has ASR you should buy differential fluid from your local MB dealer.

As mentioned remove the fill plug before draining the old fluid. Add new fluid until it's up to the fill plug hole. Either pump it into the differential, or you can remove the drain plug from the spare tire well and use clear tubing from the container through the drain plug hole into the differential. Whatever works best for you.

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Fred Hoelzle