You HAVE to use the MB fluid. The only other alternative is the same product made in Germany by Shell.
I'd buy 8 quarts, realistically it's more like 6 to 7.
There is a special tool dipstick you need to measure the level, and the MB shop computer (SDS) is used to watch the fluid temp, so the dipstick value will be accurate. MB doesn't publish an exact amount you need to put in for a fluid change, as fluid changes are not recommended.
These have scavanged cases, so the proper level is important for shift quality.