Hello,

I am surprised that no one has definitively said where the oil pressure is
sensed.  I looked in my Vanagon books, and couldn't figure it out, so then
I looked in various non-VW books to see if I could find something to
generalize from.  Only one of my books showed the oil paths through the
engine (Mazda) but it didn't show the oil pressure sensor location.

Phooey!

Some of you figured out where I was going with this sensing thing.  There
could be very different interpretations about which filter was better based
on flow depending where the pressure is sensed.

Without knowing the sensor's location, I analyzed the symptoms that Mark
reported a little further.

Assumptions:

a) The oil thins during the high speed driving, and thus the pressure
   drops.

b) Since we are dealing with low flow (idle after high speed), and low
   pressure, the bypass valve in the filter is not an issue.

c) The anti-drainback valve need not be considered either.  Only the
   flow through the filter element is an issue.

Some Conclusions:

1) Pressure sensed before the filter.

   This would seem to indicate that the Fram filter is more free
   flowing than the Mahle.  That actually says nothing about either of
   the filters effectiveness.  However, the Mahle filter may be masking
   a bad oil pump, worn engine bearings, or a bad oil pressure sensor.

2) Pressure sensed after the filter.

   This would seem to indicate that the Mahle filter is more free
   flowing than the Fram.  Once again, this says nothing about either
   of the filters effectiveness.  However, the Mahle filter may be
   masking a bad oil pump, worn engine bearings, or a bad oil pressure
   sensor.

More conclusions:

The whole oil pressure sensor design might be tolerant of the Mahle filter'
back pressure and intolerant of the Fram's.  In that case no conclusion can
be made about which is a better filter, but it's easy to conclude which is
more annoying to use.

On the other hand, the issue may be worn engine parts.

Mike