Choosing engine oil has been a serious matter to car owners. To clear this up, all that's needed is to follow the manufacturer's instructions.
For my MB, I use two different oils depending on the outside temperature. In summer it is Mobil Super XHP 20w50:
... and in winter I use Total Quartz 7000 10W40:
...as it is lighter and better for the engine. Our summers are so hot that I'm not so sure if this weight can take it, being xW40 and the engine is of older design that leans towards thicker oil in hot days.
I use these two specific engine oils because they carry the MB approval number for the subject engine and following the owner's guide's ambient temperature table.
Take care of your car, it takes care of you. Specially MB's.