From my experience with diesel pickups in the late '70s - early '80s, it's much better to use a fuel/water separator (as in a Racor unit) than to have to use chemical means to deal with moisture/algae in a diesel fuel system. When water hits the separator/filter, it swells it such that you'll notice a definite decrease in power past a certain rpm/demand range.
Modern light truck diesel fuel systems also have moisture sensors and factory water filter/separators. I don't know what VW uses on your vehicle, though, BUT you should find out. To me, this is ONE area where EACH diesel vehicle owner should become knowledgeable should they happen to get a "load" of water from a station somewhere.
The other thing is that the filter you might see on the fuel pumps on the island is a water filter, which "closes down" when enough moisture hits it, just like in the Racor-type filter/separator. Actually, the "pump" on the island is only the dispenser as the real pump is at the tank, usually. IF you notice it stop or start to run slow, that means that the tank is either empty or the filter just detected a lot of moisture. Many of the high-volume convenience store stations have electronic monitoring of tank level and moisture alarms--so the parent company knows when to send tankers out or diagnostic people to check on the "water" issue. I suspect the electronic moisture monitoring might also compensate for not having any "well" to check for fuel leaks?
In reality, the best method of dealing with moisture in a diesel vehicle is to NOT get any in the first place. The chemical means are only something of a band-aid fix, to me, but can be useful in specific instances. Buying quality fuel from a high-volume station (rather than diesel from a corner store, for example) is an observed good thing to do.
I would hope that any diesel fuel additive would not significantly decrease the designed-in lubicity of the base fuel.
Just some thoughts . . .
CBODY67
