Even with a brand new / re built motor you can
experience low oil pressure.

Check the vacuum pump by pulling it and looking
at the slot. The slot should be a rectangle ! If it is worn
(it will wear on both sides, and can wear to a “point”,
the pump will cause problems.

The vacuum pump is connected to the oil pump shaft,
if worn, it will not spin at the correct speed to make full
oil pressure.

Worn ? Replace it !

VW Diesel Parts