History, history.
The ultra low sulfur fuel has allowed for extended oil changes when doing a lot of highway driving. There is no use doing a search on this subject as it has taken years of TDIforum history to get to the point that one can run extended oil changes on a PD (brm, bew) under the right circumstances(post 2007 due to ulsfuel.
All other models do not have cam issues and can run 0w-30 if you like. 0w-30 was the hot topic back in early 2000,s and alot of members were switching to it since VW was promoting the long life.
If it wasn't for the PD we would not be using 5w-40 oil unless you pulled a heavily loaded trailer. Synthetic 5w-30 would do just fine in light duty applications.
In a PD unless you do a lot of highway travel you are going to have cam wear. Any short distance and start stop kills a less than perfectly ground cam very quickly. The franko6 cam and colt cams have seemly solved the quality problem in this regard and have improved the life expectancy. Using 5w-40 oils is required for the added viscosity due to the narrower lobes(12mm) than all previous flat tappet models(15mm)
When scurvy ran extended he did it cold turkey and used every trick in the book to make sure he was not screwing up his motor. It was costing people a lot of money when the cam problem showed up in 2008. There was no fix in 2008.
I had my cam replaced under warrantee and the second OEM cam lasted 90,000 miles. The cam ground surface was very rough and caused excessive wear on the lifters. Long highway miles may have allowed it to last much longer.