I have an '82 diesel Westy.
I also have 2 diesel Mercedes.  On them, I can adjust the fuel mixture by
adjusting the aneroid on the fuel injection pump.  These are both turbo engines
and the aneroid adjusts fuel mixture according to boost and altitude, but the
adjustment screw makes quite a bit of difference in the 0-2000 rpm range, prior
to the onset of boost.
I have never heard of this adjustment on the VW.

Thanks, please cc any comments directly to me.