Diesel oil and gas oil have big different properties

10-40 is for gas engine. although MB said 10-40 can be used in some area in winter, but that is a rare case and one should never use it regularly.

Use a local shop specializing in German cars or Euro cars. The best way, yet, is DIY if you can. Below links tell you all:

Changing oil yourself not because you are cheap

How to select a mechanic to do service and repair