Oil Filter differences in general (A4 in particular)

[Mark Quinn/HNS <Mark_Quinn@notesgw.hns.com>]

Last week I posted a question about where to get Oil Filters
for the A4 (cheaper than the $20 my dealer wants).  I got a
couple responses including someone suggesting using the Diesel
engine filter.  That got me to thinking ... what is the 
difference between ANY oil filter?  I had always assumed that
the fitting (size) was the thing, but looking around an Auto
parts store over the weekend, many (not all) filters use the
same screw dia/thread/seal dia and only differ by outside
dimensions of the can itself.  Assuming the same screw/seal
fitting and that the 'can' will fit and is as big (or bigger) 
than the original, is there other difference between filters 
by a given manufacturer (PLEASE leave manufacturer to 
manufacturer differences out of this thread!).

In this particular instance, I found a Fram PH8A that looks
close to my A4 filter.  I bought it (cheap) and took it home 
to compare to the filter I just replaced.  It IS the same 
thread/pitch and seal diameter, and the same can diameter. 
It is shorter than the Audi filter (by just under half an 
inch) but is a BIG filter anyway, so as a percentage, this 
size diff is small.  Secondly, the reason I think the A4 
filter is so big is that the oil change intervals are so 
long for the A4 (too long IMHO).  The first interval is 
7500miles, then at 15K (or 22.5K - I forget) intervals 
thereafter.  If you change oil at 3K-5K intervals I would 
assume you could use a much smaller filter (such as the filters
my prev Rabbit GTI, 4000Q and 90Q20V used to use - which 
again have the same fitting).

Well, am I going to stir anything up here?  I really am 
interested if there are any diffs in filters other than 
fitting & size (& manufacturer!).

-Mark Quinn