Oil filter inside.
 

Did a cross section cut of a used oil filter and find a spring valve inside.

http://i277.photobucket.com/albums/k...09E2DA760C.jpg
http://i277.photobucket.com/albums/k...0ACA769EFB.jpg

Looks like a standard relief valve oil well only go in with about 30psi from the looks of it in the system not sure why they would do this but I'm not to familiar with the insides of an oil filter. But from another thread is there any sand in it?

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Yup! check out the bigger subaru site if you wanna see more comparos

i talked to a tech at nissan one time (when i had my old car before the frs) and they said they have a spring inside their filters that when you start your car the spring releases and shoots oil directly into the engine to lubricate it on cold starts. don't quote me that's just what I was told.

Close, but not quite the whole reason!

The spring is for a bypass valve, the main purpose being that if the filter becomes too clogged to pass oil and the pressure increases, the bypass valve will open to let oil through, diverting the oil around the filter. The theory is that dirty oil is better than no oil at all and therefore it is kind of a fail safe. Obviously you shouldn't ever let your filter go that long so that it ever gets that clogged, but the mechanism is meant to save vehicles from stupid soccer moms who never change their oil, not car enthusiasts. It will also open during startup in some cars when the oil is especially thick and doesn't easily pass through the filter (i.e. cold mornings) ensuring adequate oil flow on cold starts. The valve will then close again once the oil warms up and thins out, ensuring oil flows through the filter. It will also open at high-rpm/oil-pressure, ensuring adequate lubrication and oil-flow under high engine load.

Do all boxer engines use this type of oil filter because they are on top compared with side mount filters?

Most car oil filters should have an oil filter bypass built in somewhere, for the reasons stated above: people forget to change a filter and a few thousand miles later they blow their engine up.

Id even go so far as saying all filters will have a bypass, or it'll be built into the engine.

Its a shameless plug, but heres my oil filter write up.

http://www.ft86club.com/forums/showthread.php?p=0682

Sent from mission control

Virtually all canister type filters have a bypass built in, it's just part of the design.

Filters however that mount upside down on the top of the engine however do have a drain-back prevention valve, which is the rubber seal you see covering the holes on the outside ring of the filter. This prevents the filter from draining back into the oil pan on startup, which prevents the engine from being starved of oil during the first critical few second after starting the engine.