According to the US VAG forums, the VAG-approved additive is Stanadyne Performance Formula.
I did obtain and try a bottle of this stuff (lasts 5-6 tankfuls) in my own PD130 B5.5 Passat (owned from new) and ..... no measurable difference in performance or economy.
The same with all the super-diesels, none made ANY measurable difference in performance or economy -- in that particular car. I often THOUGHT I felt a difference, but this was not reflected in the figures at the pumps.
Having had the Passat for over 5 years and 60+K miles, it never seemed to mind what fuel it was given. It just liked a weekly clear-out by revving to 4,000 through the gears.
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