A good synthetic rear diff gear oil is either Redline gear oil or Mobil One synthetic. They both come in the regular weight and a heavy ( truck/suv) weight) Use the heavy weight if your rear diff has the diff whine noise from a wear spot in the ring and pinion gears. It will be surprising how it will mask the whine and reduce the noise. Wont hurt the diff either. I've used both in the two weights in several MB cars. Dont use the heavy if you have no whine and dont race the car.
Mobil 1 regular weight at Autozone is about $8 a quart, $16 a quart for the heavy gear oil. Red Line is harder to find, typically for racing and is a good all synthetic, more expensive and likely not any better if you dont race the car.

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Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red