As I recall, my 2001 C came with a booklet of "factory approved" oil brands. It also came with a bunch of other stuff urging us to use Mobil 1.
I glanced at the list of "other" oils approved by MB, it seemed to be many, but not certainly not all, of the major brands. For example, I don't think Pennzoil was on there.
Anyways, I didn't pay much attention, since as I understand it, Mobil 1 is the factory fill, and it is certainly what they encourage you to use. So, I just took their word for it and gave only a passing glance to the "other" oils.