I was looking around the Subaru Endwrench site (http://www.endwrench.com/) and happend to find this http://www.endwrench.com/current/win...03/insider.pdf
It would seem that the only difference between the two filters is that the H-6 has about 40% more surface area.Oil Filters: H-4 Versus H-6 Engines
Subaru H-4 engines come from the
factory with an OEM P/N 15208AA060
oil filter. The same part number is used
for replacement. H-6 engines come
with an OEM P/N 15208AA031, and
S0A5165109 is used as a replacement.
Both filters have the same physical
dimensions as far as threads, O-ring,
and bypass valve opening pressure (23
p.s.i.) are concerned. However, the H-4
filter has 141 square inches of filtration
surface, while the H-6 filter has 201
square inches of filtration surface.
The H-4 filter should not be used on
the H-6 engine. Using the H-4 filter
may cause it to become contaminated
faster and allow the bypass valve to
open, allowing unfiltered oil to circulate
through the engine.
I'm going to change my oil today (gotta get ready for Carlisle ) and I'm planning on using a Pure One filter for the H-6 (P/N PL14459). I'll let you guy know of the differences, if any, and how it works out.