stephenson:

If you take a look at MB's approved oil list for new cars (from 98) and recommended for old cars, there is no oil that is of the x(x)W-30 variety (among these available in the USA). As a matter of fact, it specifically says that Mobile 1 Tri-synthetic 0W-30, 5W-30 and 10W-30 should not be used (too thin).