Well, shorter FCIs in a bypass system obviously will introduce more fresh oil, due to volume replenishment. That is different from true "consumption" via evaporization and/or burning in the combustion process. That is why I brought up the topics seperately. Together, they become "make up oil". Sometimes you can make judgements of the oil based upon the total make up oil versus the consumption of oil. Oil that resists evaporation and oxidation will have little affect on the "make up oil". Also, oil consumption is greatly affected by engine loading, especially in a diesel. How much you add to overcome oil "burnt" is different from how much you add to replace filter volume. That's fairly obvious.
What I see as a valid concern in your question is how do we attribute great performance to the oil when so much of it might be "new". I guess what I'm telling you is that some amount of oil has to be new by necessity of operating a bypass system; it simply cannot be avoided.
The longer your filter change interval, the lesser your need to replace the filter capacity. Duh - right? So, with bypass use, you are always playing some fundemental game of seeing just how long you can run your bypass filter element. Sometimes, the only way to know when it needs to be changed is by touching the element canister, and seeing if it's hot to the touch. Cool temps indicate that the flow is so poor that the filter is basically plugged. This is not a danger at the bypass element because it has no bypass valve like the full flow filter necessitates for safe operation. However, this clearly is a low-tech gage to know when to change a filter.
This topic is, at it's core, just one of the many attributes one has to consider when reviewing the choice of using bypass filtration. Consider the bypass filtration with TP; oil make up via filter element change is a HUGE part of overall cost. The toilet paper is cheap; it's the oil replacement that kills the cost ROI. And that constant replenishment gives you a nearly "new" fresh oil system very frequently, because the sump's total capacity is greatly affected by the bypass system by percentage.
Bypass filters on smaller sump systems stuggle to make fiscal sense, and they also create a great affect on the oil "freshness". Very large sump systems don't become effected as greatly, because their percent volume is skewed to a much smaller degree. In larger systems, the pay off is by volume reduction at purchase time. A 10 gallon sump is much cheaper to top off than replace. A 2 gallon sump is pretty darn cheap to replace. Since the "top off" rate is fairly constant (cost of filters and amount of oil used) you can find whether it's cheaper to use bypass, or just dump/fill a "normal" system.
So, back to your initial comment/question, it's true that bypass systems do somewhat skew the view of oil performance. They have to, to be able to perform their foremost job, which is to greatly extend the OCI. I suppose you could put on a bypass system, and run it for as long as possible before you had to add oil or change filters, and then do a UOA. At that one point in time, you'd really know how well the oil was able to last. But after you top off, you have manipulated the original load, so the "virgin" oil load is now no longer "pure". Then you add oil and change filters. Now, you've no longer got a virgin load of oil. But keep in mind that the MAIN driver of bypass is to save money. You save that money by purchasing less oil. As long as your oil in the sump is viable (regardless of age) then it's serviceable, and cost effective.
Running a bypass system isn't so much about praising a particular brand of oil, as much as it's about complimenting a well-managed system. The engine, the oil, and the filters all acting together are manipulated to achieve the greatest ROI. Utilizing a bypass filter often makes any oil look good by UOA results. But it's really about the OCI duration; that is the goal. As long as your oil is in good shape, it is worthy of "praise", but it's really the filter that got it there. The goal of bypass filtration isn't healthy oil for the sake of the oil bragging rights. The goal is healthy oil for the sake of your engine, while making your wallet happy!
Further, a little considered fact is that one of the "best" uses of bypass filtration is actaully with dino oil, rather than synthetic oil. Here's why ...
Presuming the bypass fitler is capable of keeping ANY oil clean, then why have the expense of synthetic at 3x the cost? You can get a good quality dino HDEO for $2.50/qrt, versus $7.50/qrt for synthetic HDEO. So, presuming your total make-up oil is a low percentage of consumed oil, versus filter capacity replenishment, then why would you run an oil that costs 3x as much money, when it's the VOLUME you're replacing? If you have to top off fluids MOSTLY because of filter capacity (and not consumption), then where's the logic in spending 3x the money per quart? It's likley that the oil load will be viable for the same duration. The bypass filter keeps ANY brand or grade of oil at a very clean level. So where is the "advantage" of synthetic oil when the dino oil is just as clean?????? Once you top off with an HDEO (regardless of base stock) your add-pack is bolstered! So where is the sense in spending the extra money for synthetic?
Here is a quote from me in an earlier post in this thread:
"My synopsis: fantastic dino oil, great filtration, very reasonable oil consumption, and a probable Cu mistake."
That is not just about the oil; it's a comment on the system management as a whole. Arkapigsiesel is very diligent about his ride, and puts a lot of thought/effort into maintaining his truck. He is very versed in bypass filtration, as he's on his second brand of system. He's done PC analysis in the past, as well as UOA analysis. So, my statement of "fantastic dino oil" was inferring that the oil (Delvac 1300) is known to be a great oil right out of the jug; it was not a comment on the UOA results as the oil being a single contributing entity. Rather, the WHOLE SYSTEM is responsible for the great UOA results. (Aside from the Cu, which is likely a error). Does that help make better sense of it? Perhaps I gave a poor impression initially. I was complimenting a well maintained system, not only the oil.
The key is to find the balance point where you have the least affect (manipulation of input), while realizing the greatest effect (resultant ouput)!