That black coating looks a lot like the black coating that you usually find all over everthing when you remove a diff cover to change the diff fluid. I'm sure that it is fluid related. On the diffs, it is not supposed to be harmful. Remember that usually diffs have stronger additives, especially ep (extreme pressure) additives to put up with the higher pressures from the hypoid cut gears. Some of these ep additives are what makes diff gear oil unsuitable for manual transmission use.

I wonder just what MTL has in it.

Jim Johnson 98 SVT 03 Escape Limited