The fluids get greyish black in these because the wear part inside are made from graphite and discolor the fluid. If it does not smell burnt then the chances are it is fine, but changing it will not hurt from what I have heard. Use only the MB SYNTHETIC fluid, use a new filter, drain the TC from the plug and you should be good to go. Since the material and special tool to read the level will cost about $125 to do it yourself you might just let a good indy shop or dealership do it, even if it lands up being $200 or more. They know how not to overtighten the bolts and where to check for leaks (at the electrical interface plug) when they are working on it. Just my $.02.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz