I use T6 on my diesel Jetta as it has a dry turbo which is prone to hot shutdown heat soak. Also heard it has an issue of coking oil on piston bottoms and top ring grooves. Been using it for last 100+k miles, did a sample, came back ok at 10k interval. I think it is good stuff.
On my boat 450C Cummins, I still use dino Rotella 15-40. It has a wet turbo and never gets shut down quick from a hard run. I also only cruise it at max of 220hp, which should not get piston temps up where coking is an issue. I think. At about 4000hrs on it so if there was an issue, I'd probably have seen it by now.
Oil sample at 250hrs on the Cummins showed the oil was fine for continued use, but I end up changing it anyway at around 300hrs just because running beyond that puts a nag in my mind, and I start thinking about it.
The VW definitely specs a syn. The Cummins does not, so there is that too.
This is highly non-technical, but it seems some engines "like" some flavor of oil over other flavors. So if your engine is doing something weird, like using oil, nothing wrong with trying another flavor.
Going with syn does not automatically mean you can extend change intervals. Crud added to good oil still ends up being cruddy oil. Sampling is what tells the story. Even dino oil can go well beyond spec change intervals, and it has more to do with engine duty cycle and other stuff, not so much whether dino or syn.