Conventional Motor oil VS Synthetic for Diesels

I would say the point is sort of moot. Mercedes has a distinct spec for the oil, and it may not be wise to use something else, either due to particulates, or other properties.

For other vehicles, I've found that synthetic does last somewhat longer. Generators, it actually does a bit of good making them a tad quieter, although that could be imagination. However, since generators are splash lubricated, the oil has to work harder, so might as use synthetic there.