Hmm, I just responded on the other dealer oil change thread Stone...
Not sure you'd notice a giant decrease in oil pressure if the bypass valve opens up because it's designed to keep the pressure high "hell or high water/debris or not" by allowing the oil to flow past the filter element when it gets plugged.
Are you on a new engine break in period right now (fresh main bearings, new con-rod bearings, rings, etc...)?
I once had an experience with a high-filtration no-bypass (Canton) filter where I put it on my Cobra too early after a fresh engine build. What happened was the bearing babbitt plugged up the filter a bit and I noticed the pressure began to drop. I shut it down, nothing seemed to be wrong, and upon restart the pressure was once again fine, but as I ran it some more the babbitt began to get swirled around inside the filter body. The filter again began to plug and the pressure again began to drop. I shut it down, same thing again, nothing looked wrong; restart, good pressure at start, then slowly dropping.
When I opened the filter, it was obvious what was going on. I did a few hundred more miles with a Motorcraft filter to clear all the break in crap out of the engine, then went back to the Canton no bypass type. No problems since (72PSI on cold start, 45-60PSI on hot/full temp running motor depending on the revs).
Do the flakes in the oil attract a magnet? If it's bearing babbitt, that's is usually soft metal alloy with some lead which doesn't necessarily grab a magnet very well.
Is the used filter exceptionally heavy,more than it would be just being wet with oil? Cut the old filter apart and see if there are large pieces inside the filter body, possibly holding open the anti-drain valve or bypass valve (see my other post to see what those look like).
Personally, I wouldn't worry about the anti-drain valve too much. For a filter which sits vertically the anti-drain flapper doesn't do anything really. I don't even install them anymore when I replace the element during an oil change on my Canton type.
Definitely get the oil analyzed. That should tell you exactly what's going on...
