I can safely say that I've done my share of research on this subject, as I wanted to make the best decision for me factoring in the questions you raised plus some you didn't.
I've talked to Cummins, lubrication engineers, the folks at Lubrizol, the makers of the additive packages for Mobil/Exxon, plus some others.
The Amsoil dual-bypass has some problems, as I have found quite a few people that have taken theirs off, and installed either the single-pass Amsoil unit, or an equivilent unit like the Oilguard or Filtration Solutions.
I'm not sure what all the problems were, but this is the impression I was left with.
If you're going to change your oil at the recommended intervals in your manual, don't waste your money on bypass filters or synthetic oil. You're wasting money. Use a good filter like the Fleetgard Stratopore, and you'll enjoy very long engine life.
If, on the other hand, you intend to keep your vehicle and rack up a lot of miles, then switch to a synthetic oil. Amsoil is good, so is Royal Purple and Mobil Delvac One.
Mobil Delvac One, BTW, is the only oil that Cummins has personally tested "cradle to grave", as they call it. I personally think Royal Purple and Amsoil are a better quality than the Mobil, but that is from a lot of feedback from oil analysis boards and testing results. All three are excellent.
Your normal engine filter only filters in the 30 - 50 micron range at best, and that's because your engine requires a flow rate somewhere in the neighborhood of 10 gal/min, and if they filter to a lower level, your engine would be starved for oil.
Bypass filters will filter in the 1 - 50 micron range, but they have typically an "absolute" rating. Don't be fooled by this 1 micron business. Look for the "absolute" value, if you can find it. The "absolute" value, like on the Filtration Solutions model, means nothing above 2. 78 microns will pass through the filter at any time. I'm not sure, but I believe the Oilguard has a 5 micron "absolute" rating. Anyhow, any of the three are good units, and will remove all the harmful soot and debris that will harm your engine over time. They also will remove additives, but not as a filtration function by itself. The additives are designed to "bind" to the material that is harmful, to make it bigger and hopefully be caught by your regular filter, which only happens for the bigger elements. The bypass filter will remove, for the sake of argument, everything down to 2 microns, which includeds any additives bound to the contaminants. These additives are replenished when you change your bypass filter and add, typically, a quart or more makeup oil. And yes, the contaminants that cause the most damage to your engine are in the 3 - 10 micron range, small enough to get in between sliding bearing surfaces and grind away on whatever they touch. So that's the advantage of the bypass filter. With a bypass, and regular oil analysis, and regular bypass filter changes, many users can go over 100,000 miles before a major oil change is required, although your regular filter will have to be changed somewhere in the halfway point, about 50k - 60K miles.
On the oil pressure business, there could be several possibilities. I have heard similar complaints from others concerning the Amsoil dual-bypass, as well as the more seriously flawed Fleetguard bypass. In Fleetguards case, it's because the bypass extracts too much oil through it's unit, resulting in low pressure. Sort of like your blood pressure when you slice open an artery. OTOH, there are no complaints at all from the Amsoil single bypass users that I have found.
Anyhow, this may be more information than you wanted, and the conclusions and opinions I present are strictly my own
Hope that helps a little.
Robert
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