These special engines, the VW pd engines, are very severe on the oil, the mechanical fuel pump delivers fuel at approximately 25,000 psi!! to enable a very precise injector spray pattern which, in turn, gives excellent driving characteristics.
The very high loads on the fuel pump will lead to VI improver shear down and oil viscosity loss if the engine is operated on lesser oils that ones approved for use in PD engines.
Cheers
Guy