I'll chime in here; been a while since I've touted, so to speak.
I firmly believe that bypass filtration is benefitial; it helps extend your OCI, and that's can be a huge fiscal savings tool if you can keep your (joyfully) grimey hands off the wrenches.
However, there's a lot of sensible logic, and analitcal data that suggest bypass filtration does little to add "life" to a piece of equipment. Now that I've opened the can of worms, let me explain where my thoughts come from.
Many years ago (let's say 20+ for the sake of the argument), cars and their engines didn't really last that long. It was very easy to add life to the engine with a bypass filter, as the full flow filter wasn't that great at the time. Further, the engine build quality was nowhere as good as it is in the last ten years, and continues to get even better.
But now, build quality of the engine, and the materials selected, and the design, along with better full flow filtration, can easily make an engine last 250k miles if just OCI'd properly.
Consider that the damaging particles a UOA can see are all 5um and smaller; that's the approximate limit of spectral analysis in a UOA. Yet, we also know that most engine damage occurs in the 5-15um particle size range, most frequantly at start up. Now, while it's possible that particles can start off this large (they break off in a landslide, and throw chunks into the stream), this it NOT the most prolific of chances. It is MUCH more common to have a large quantity of smaller pieces floating around, than to have several big ones. UOAs cannot tell us the size of the particles, only the composition; we can assume they are smaller than 5um, because that's all it can see. But particle count analysis can tell us both the size and frequency, but NOT the composition. A conundrum, we have.
As far as soot and wear metals, it's far more likely that they start out small; for soot, it's even typical to be sub-micron in size upon birth. So, until those little particles co-join (agglomerate) and grow, there is little damage that they do. They have to get 5um or larger to do any real damage, and by then, you are loosing sight of them in the UOA. Anything that becomes too large with immediacy is likely going to end up in the full-flow filter asap; the risk is low that it would continue on. The real danger is those 5-15um particles on their constant journey.
Here's where pragmatic logic occurs. The "best" way to keep an engine (or any other lubed piece of equipment) healthy is to keep the lube system clean below the critical level that the equipment needs for a long healthy life. For an engine, that's typically below 5um. So, we can achieve this one of two ways.
1) super filter the lube with bypass filtration
2) change oil often, before the particles become to large.
Either will achieve the task. You either clean the oil, or replace the oil. It's just that simple.
There's a member here (ottomatic) that had a 6.9L IDI Ford Diesel that had 450k miles one it before it was stolen. He OCI'd every 4k miles with dino HDEO 30w. 450K miles!?!?! Gee - just how much longer would that engine have lasted with bypass filteration??????? Who really knows. You don't. I don't. My point is that he OCI'd often enough to keep the contaminants down below the critical level. In fact, in theory, you could never use a filter at all, and if you changed oil often, you'd still see very little wear. I wouldn't advise to do that; they cost of a bad incident could negate the savings very quickly.
There are other benefits that people would claim regarding bypass filtation, such as:
1) increased system capacity resulting in lower contamination percentages
2) increased system capacity resulting in cooler oil
Let's consider these claims; I would agree that both are true. But neither is UNIQUE to bypass filtration. I can add a larger pan and get more oil, resulting in cooler oil and a lower PPM concentration. I could add an oil cooler that would do the same. I could add an additional full-flow filter system and do the same. In fact, ANYTHING that adds capacity, adds these benefits. So how is that unique to bypass filtration? It's NOT!
Here are two examples of how frequent oil changes can result in longevity. Neither used fancy filters or oil; just fequent oil changes.
http://www.knfilters.com/news/news.aspx?ID=157
http://www.autoblog.com/2008/02/01/wisconsin-mans-91-silverado-set-to-hit-1-million-miles/
Both these guys just changed oil every 3k miles or so. So, you see, it is possible to get extreme life from a vehicle, without having extreme filtration.
The ONLY thing, repeat, the ONLY thing, that bypass filtration does in a
unique manner is clean the oil "better" for "longer". That's it. Period. It does not make the equipment last longer.
So, if you change your lube often enough, you can negate all but one benefit; you cannot save money by changing oil frequently. That is where bypass filtration has a UNIQUE advantage; it prolongs the lifespan of the oil.
Bypass filtration is a fiscal savings tool for your wallet, not a fountain of youth for the engine.
Equipment lifespan is effected by proper maintenance routines, much more than super-duper products.
My belief is that bypass filtration does not add life to your engine; it extends the life of your lubricant.