Synthetic and synthetic blend oils, as long as they are the proper spec, aren't bad for your engine. They are far worse for your wallet than they are for your engine.

I believe a lot of these opinions are formed from mechanics and owners who may have experienced increased oil leakage with synthetics. As most everyone knows, synthetics flow better at a given temperature than conventional oils. This is one reason why a lot of people living in cold climates use synthetic oils, especially in winter. It makes their cars easier to start and their cars seem to run better, particularly during warm-up. This ability to flow better may also mean that it can get around seals in your engine. So, if you have a minor leak with conventional oil and you put in synthetic oil you might find your leak to be a little bit worse. The next issue is that old Mercedes diesels are known to leak. Personally, I don't believe that Mercedes diesels should be considered leak-prone by themselves. It's probably more related to the fact that Mercedes diesels tend to last longer than many other engines. Seals do age and wear out. If you could check a decent sized sample of 20-year-old gas cars that had 300k-400k miles on them (assuming you could find them) I bet the frequency of oil leakage would be similar to our old diesels.

The point I suppose I'm trying to make is that, as long as you don't have a leaky engine and you don't mind the additional cost, you can certainly use synthetic or synthetic-blend oils. I wouldn't try to extend the drain intervals though, especially with the stock oil filtration system.