Oil Filters and Anti-drain back valves

["Dan Sinclair" <sinclair@prospro.com>]

Quick question for the contemplation of the list.

In the Audi 2.3, 5 cylinder (Audi 80 and 90 anyway) the oil filter is
located at the absolute bottom of the engine in an "end down" orientation.
It is virtually level with the oil pan.  My question is:

How would an anti-drain back oil filter keep any significant quantity of oil
in the upper portions of the engine? (as noted by listers and others, this
anti-drainback valve would help keep oil from draining away from vital
engine components during start up).  Whenever I've changed my oil, a small
quantity of oil does come out of the filter feed tube - is this a function
of the anti-drain back oil filter?  If so, that doesn't seem like enough to
make a huge difference, and in fact, it doesn't seem like any more than when
I've used a non-valved filter.

It's my understanding that ALL filters only "filter" small portions of the
oil at any given time, the full quantity of oil is not passed through the
filter in continuous flow... so how much advantage are you getting from an
anti-drain back filter in an engine with a bottom mounted oil filter?  I'm
not suggesting that the technology is not useful, I'm just curious if others
have contemplated the logic.  I can see where on my old Toyota, with a very
high, block mounted filter you'd stop quite a quantity of oil from draining
out of the block, but does this hold on the Audi configuration?

Thanks in advance

Dan Sinclair
1988 Audi 90 67K