I recommend putting an an analog oil pressure gauge.  I wrote it up
when I did it and it's out on the Vanagon.com site under problems.
That way you know whether it's really the pressure or something else
wacky.

On our engine, which is getting close to 180K miles, I get low oil
pressure when the engine is hot, it's hot out, and I'm idling at a
light becainse indeed, I have about 10-12 psi with 20W50.  I never
get the oil light coming on when it's cold and the pressure pops
right up as soon as I hit the gas.

Details: Mahle filters, 20W50 (no brand preference), '87 GL.

My $0.02.

-Karl

Karl and Kristina Bloss, Trexlertown, PA
'87 GL Weekender  "Beverley"
VW shop list: http://www.enter.net/~bloss/vw/vwshops