This is not good.  See my article on Tom's site:
http://volksweb.relitech.com/21rodbrg.htm

It could be a sender problem or your rebuild may not have been as tight as
it could have been, which is why I like to track oil pressure right from the
installation of a new engine.

If you are in doubt, install an oil pressure gauge to find out for sure.

Stuart MacMillan
'84 Westy 

Original message:

From:    Eric Zeno <vw4x4@FYI.NET>
Subject: BAD OIL Again!

DAM!   Last summer I was changing oil in my 1986 Syncro,
and put in Mobil regular oil.  To make a long store short,
I had a lot of trouble with the 20-50 weight oil being to
thin when hot.  It would set off the oil light and buzzer.
    Once again, 1 1/2 years later I'm having the same problem.
Start it up cold, no problem, 15 minutes later, oil pressure
problems.  Is my the engines going bad?  Or maybe the
engine oil is changing and will not run in the vanagon
engines anymore?  I can't imagine my GEX rebuild was
going bad 1 1/2 tears ago and now later it is finnally
getting worst.  I'm changing the oil  and filter tonight
prematurelly.      Comments?
                                          Eric