MB and BMW still specify thicker oils than many manufacturers. BMW even formulated a special 10w-60 for their high performance M engines. MB current service literature supercedes all owner's manual and previous viscosity charts and oil specifications.
MB's highest rated oil currently is 229.5 (e.g. Mobil 1 SuperSyn 0w-40) for both gas and diesels. Other current specs are 229.3 and 229.1 (Mobil 1 SuperSyn 15w-50). If one cannot find an oil meeting MB specs, then MB recommends ACEA A3/B3 specs and failing that, the use of API Sx/CH-4 spec of an approved viscosity as a last resort.
I believe my 1987 owners manual allowed 5w-20 or 0w-20 - but only in arctic conditions. The current standards specifies higher viscosity oils such as 0w-40, 5w-40, 10w-40, 15w-40, 15w-50. Where the 0w-40 and 5w-40 are "all season" oils.
I won't second guess the MB engineers as to their reasons for these viscosities. If you want to follow GM engineers, then drive a GM car.
Lube in peace,