Before I ask my Q?, I want to state that a member here has sent me PM's calling me a moron. I guess my sense of humor has labled me as such. I apologize.
I have seen/used oil filters with a double by-pass (I'm almost certain they have a double by-pass). Why would an oil filter have a double by-pass? In case one were to fail? They are set to open at different psi?

Technically speaking there is a normal anti-drain back valve with an anti-siphon valve. The anti-siphon is in the center tube and seen near the outlet hole.

Iirc, the Ford Zetec engine specs a filter with that set up. In addition to adbv, the anti-siphon valve apparently needed to help assure oil remains in the filter.

There is also a separate dome bypass which performs it's normal function.

All it does (like the supertech ST8316) is keep the entire filter full of oil

Is this aimed at oil filters that are inverted, AKA-dome end facing up, or on an angle 90* or more?

I like your questions and comments BlueOvalFitter. Some id10t is trolling you. Ed

That would be logical reasoning.... I think the ZETEC had them horizontal

Linctex,

I checked the Wix filter site and they list a 51315 filter. But it appears to just be a regular filter. Baldwin, Fram and Purolator do not cross reference to a ST 8316. I'm having trouble understanding how this filter works. Along with a regular anti drain back seal / valve and bypass valve, as found on most filters, is there another one way seal/valve inside the center tube that keeps the oil in the filter?

Yes there is. It can be seen in slacktide bitog's linked TG 8316 c&p.

https://bobistheoilguy.com/forums/ubbthreads.php/topics/3866309/Not_bad_for_a_cardboard_Fram!_

As for the OT trolling accusation, not sure who that would be in this thread as imo and observation seems all answers put forth in an honest attempt to answer the topic question.

Yep, just a low pressure check valve in the bottom of the center tube to prevent the filter from siphoning/draining out the base center hole. Think of it as the center tube ADBV.

Apparently some engines can drain the filter from both the filter inlet and outlet.

Maybe so, but I don't see how, considering that healthy ADBVs seem to require a little pressure to open them even in the forward flow direction. Maybe if the filter is mounted high there'd be enough hydrostatic pressure difference to open the valve and allow oil to siphon out. ??

A friend's Audi TGDI uses an oil filter with both ADBV & anti-siphon valve to minimize delay in oil supply for the turbo. The Fram model number is PH / XG 10600, Mobil 1 is M1-404. This particular filter mounts vertically dome end up on the top side of the engine. It has a dome end BPV.