My results, your may differ...
I have had a single Amsoil bypass filter installed for about 4 years and around 80,000 miles. I have not changed the oil or the bypass filter yet. I can monitor the bypass flow because I installed the return in the filler tube. To check, I open the cap and look.
My last analysis shows <1. 0% soot, TBN of 12. 8. The silicone was up to 15 ppm (average before was between 6 and 7) but I atributted this to some sealer that was exposed to the oil on a repair I'd done (DON"T use sealer on the timing cover!!!!)
I mounted my filter to the back of the front crossmember (under the radiator) in a horizontal position so it is protected against debris. I laid out everything and used string to determine the hose lenghts I needed. I had a local hydraulic shop make up the steel braided hoses (use swivel fittings on both ends to make it easier). Then I covered the hoses with split loom (the stuff that is on wiring harnesses) to keep it from chaffing everything it touches.
I bought the filter base and return line fittings (where it goes into the filler neck) from Amsoil.
I started out using Amsoil 5w-30, then began varying make up oil between the 5w-30 and 15w-40 depending on temperture. I initially had a pretty good front seal leak which required a lot of make up oil. Just before I repaired it, I had to add a quart about every 2,000 miles. Sometimes more, sometimes less, depending on trip length.
At this point, I'm hundreds of dollars ahead of frequent oil changes, not to mention the time and disposal issues. Initially, I thought I would be behind $$-wise in the long run, especially with the leak, but that isn't the case.
I do change the full flow filter every 12,000 miles or so and now that I only add a quart of oil every 4,000 miles, I'll have to do more frequent oil analysis. The bypass flow is begining to slow down so it's time for a change there too.
Sorry for the book, but that's my story...