Interesting, manufacturers state lots of differing guff about oil. True fact is mineral oils are getting harder to obtain, some large vacuum pumps run on a straight mineral 30 grade, this is starting to become rocking horse sh-- you can still get it but at a price.
We changed over to part synthetic on our diesel vehicles two or three years ago, and that did not take into account how old or how many miles the engines had covered. There have been no ill effects to date, it seems to be general practise to use part synthetic in smaller garages, I can't speak for the main agents I would think they are on full synthetic.
Mercedes have been using full synthetic for years, we had a new Mercedes van and after its pdi the next service was at 55,000k, reason given it was running on full synthetic and did not carbon up as it would running on mineral oil.
Go figure as I just don't know.