A few things I'll throw in here since I did replace the differential fluid in my C55 AMG earlier this year before I totaled it . I did a fair amount of research on this on MBWorld.org Mercedes Benz Enthusiast Community and as others have mentioned, although Mercedes-Benz maintenance guides do tend to either (a) neglect to mention differential fluid change as part of regular maintenance, or (b) state that it is maintenance free and does not need to be changed, many enthusiasts opt for the safe route and I knew many C32/C55 owners who insisted 40,000 mile intervals was a good rate at which to swap the diffy fluid.

That being said, I took this advice and went ahead and drained and refilled by C55 AMG differential and when the fluid came out it looked really clean actually! Still hadn't lost its synthetic clear color.

A few things to be careful about.

(1) I don't think anyone here has actually mentioned the OEM recommended differential fluid viscosity. The MB recommended for the C55 was 1.5L of Hypoid Gear Oil 85w-90. I actually bought the Mercedes brand at $32 a liter to be safe. It very well could be different for the SRT-6 or Limited, but all I am saying is just don't go out to Advance Auto Parts and buy the first synthetic gear oil you see. In doing research I found a thread in which a guy filled his differential with the wrong fluid and it turned to molasses once he started driving around killing his gas mileage. Needless to say, he had a heck of a time getting it all out!

(2) Here were the tools I used to change the fluid:

1.5 L Hypoid gear oil 85w-90 (bought two quarts from autohausaz MB brand for $58)*
14 mm hex socket
Breaker bar (optional but I bought one out of precaution, the filler and drain bolts are hard to get off)
Liquid transfer pump (to fill diff.)
Good jack stands or ramps (car needs to be level)

(3) For some more good reading check this thread out: Differential fluid change DIY? - MBWorld.org Forums

Anyone know what the OEM spec viscosity is for the Crossfire differential?