Yes, Dexron III was specified for pre-2005 GM cars, and the newer THINNER Dexron VI is specified for the newer GM cars. Viscosity index on the VI is about 10% thinner than the older III.
Does this make any difference on the older MB cars like my '91, that specifiy in the owners manual that you can use Dexron III - I don't know, but I will stay with the III as long as I can find it.
I found Pennzoil Dexron III at Pep Boys last week.

DG