My 2004 owner's manual states "Only use synthetic engine oils, approved to MB 229.3 or
MB 229.5, such as Mobil 1 SAE 0W-40." So the requirement (for warranty coverage and acceptable operation, etc.) isn't that I use Mobil 1 0W-40 or some 5W-40 that the shop says is a replacement or compatible. The requirement is that I use an engine oil that is approved to MB 229.3 or 229.5. On those lists there are 0W-30, 0W-40, 5W-40, 5W-50, 5W-30, and 10W-40 products. And, worldwide there are more than 200 products that are approved. There's probably little need to deviate from approved 0W-40 or 5W-40 oils which are readily available, and probably provide the best blend of overall performance, but if MB is telling you that Mobil 1 FS 5W-30 is approved, for example, there is probably no major issue in using it. On the 229.3 list in particular there may even be some oils that are synthetic blends which might not totally agree with what is stated in the owner's manual.