There are quite a few 5w40s on the Mercedes 229.3/229.5 spec list, so I doubt it's going to be an issue as long as it's formulated properly.

The Mobil1 thing is a US marketing partnership - Mercedes don't recommend Mobil1 in Europe and as a matter of fact I'm not even sure it's widely distributed there.

The only Castrol 5w40 I've ever seen is Diesel Formula which is probably wrong (doesn't meet 229.3, last I checked), but maybe there's another one which works.

This has been discussed to death elsewhere on the forum anyway...