I just changed my differential oil recently. From the looks of the oil, it hadn't been changed in a looong time. I got the 14mm allen from AutoZone, only about $8 for the set of 11mm, 14mm and 17mm. I needed a 6" extension between my breaker bar and allen to go around the exhaust. Although as mentioned, synthetic has not been MBZ approved, I used Mobil 1, 75w-90. I managed to just use the tall thin Mobil container to fill the oil. You need to really crush the container to squirt the oil in and then refill the container from your second quart. Takes about 1 1/2 quarts.

glenmore
1991 300CE