MB purists will tell you that you must use MB brand but I've had Valvoline SuperSyn in my ML430 for about 6 months and in the ML320 for about 1 month. So far, no issues and they're both actually quieter. The MB brand is NOT synthetic.
Hello Mitch. Looks like that is actually approved by Mercedes.
Here's the list:
Approved Dexron III Automatic Transmission Fluids for M-Class W163 Power Steering
Castrol Dexron-III/Mercon ATF F-30341
Castrol Transmax M-22257/22096
Chevron ATF Dexron III F-30108/30159
Citgo Multi-Purpose ATF D-21571
Exxon Superflo ATF Dexron III F-30111
Havoline ATF Mercon/Dexron-III F-30321
Texaco ATF Mercon/Dexron-III F-30321
Pennzoil ATF D-22413/F-30110
Quaker State Dexron-III/Mercon F30161
Sunoco ATF Dexron-III/Mercon F-30176
Unocal Multi-Purpose ATF D-22413/22431
Valvoline ATF Type D