Oil Warning Buzzer - Possible Causes (Oil Pump, Oil Filter, Vacuum?)

[John <jb@access.digex.net>]

I recently started having my oil warning light/buzzer going off again in
my Audi 4000SQ.  The first time it happened was a year ago, after not
driving it for months.  (Out of town.)  I changed the oil and everything
has been fine - UNTIL NOW.

This time, it started happened on and off, usually during acceleration.

It buzzes for about 5 seconds, then goes away.  It usually happens when
the car is colder, and only every 3-4 times I drive it.  It will occur up
to 5 times within a 30 minute period.

I have opened the oil cap with the engine running, and see oil splashing
around in there.  My oil stick shows full.  Tried changing the oil w/ 5
quarts of 10W30 and a new filter.  Still have the problem.

Am I correct that the buzzer goes off due to low oil pressure?

My unknowledgeable guesses as to possible causes:

1) Oil pump going bad - but Linda @ Carlsen claims this almost NEVER goes
bad.  It's also $320.

2) Oil filter causing too much resistance - I use filters with the
anti-backflow valve (Bosch, AC Delco) - Should I try a Fram, which does
not appear to have the valve?

3) Oil pressure detector malfunction - Some of my vacuum lines are old.
Can this be the cause?  Does the oil pressure check even use vacuum lines?

Any suggestions?

Thanks (yet again) guys!

John
jb@digex.net