Mercedes doesn't have any recommendation to change the fluid. That being said, change it whenever you feel like it (maybe at 100k miles).

Not to be smug, but if you need someone to post a picture of a differential so that you know where it is and what it looks like, you may want to consider taking it to a mechanic if you want work done on your car.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record