In regard to the TBN/TAN, that is a topic where not everyone agrees, probably because they really don't understand the relationship, or the reasoning behind it.
You cannot only look at TBN alone; never gives you a full picutre. Here's why ...
TAN will increase over time, but just like the depletion of TBN, the increase of TAN is not linear; it's parabolic.
TAN may likely cross over TAN; often will at some point, if you run out your OCI long enough. But, that does NOT indicate that something is wrong or bad.
When both TBN and TAN are reasonably low, then there is little cause for concern. These are predictors of lube health; they are not the tellers of equipment wear. They can only predict what MIGHT happen, should things continue on a developing parabolic path. Just because TBN is low, does not mean TAN will automatically be high. Even when they cross, that still does not mean TAN is high enough to be a reason to condemn the fluid. It's a cautionary marker; it's not a absolute assurance at lower magnitudes.
Generally, Blackstone warns of TBN being "low" at 1.0; they warn of TAN being "high" over 8.0. As long as you're within those thresholds, you're probably OK. As you approach those limits, you need to pay closer and close attention to your wear metals.
Why are we concerned about low TBN? Because TBN offsets TAN. But low TBN is not an assurance of high TAN; it's only a risk indicator of high TAN. TAN will often cross over TBN, but still not get high enough to cause acidic degradation of metals in the engine.
Wear control is the paramount purpose of lubricants. While they do "clean" and "cool", their primary purpose is to reduce friction which reduces wear. That's the simplistic view of it; and it's a pretty good overall concept.
You need to monitor TBN/TAN when you extend an OCI. When TAN crosses TBN, it is not an assurance of wear escalating; it is only a predictive event that means you need to start paying closer attention to wear rates, so that you can discern when wear is being effected. Many folks use the cross-over event as a condemnation point; that's a "play it safe" action. They are not doing an OCI because wear was effected; they are doing an OCI under a preconcieved notion that wear will eventually be effected, but they cannot predict when that would occur. Only wear rates tell you when wear is effected.
I have had some UOAs run out long enough where TAN crossed over TBN, but when you look at the wear metals, you'd never have known anything happened. The cross-over event never effected wear at all, because the cross-over occured at a low magnitude, not a high one. While the base was low, so was the acid. Hence, no destructive acidic reaction.
Using the TBN/TAN cross-over is not really any different from using the odometer to OCI; they are only predictions based upon supposition, and not actions based upon factual evidence of wear.
See the difference?
Conceptually, you have to imagine an overlay of the two parabolic curves interacting with each other. Only when TBN is low, and TAN is high, will you need to then concern yourself with watching wear effects from acid. (Acid wear is a different cause than mechanical wear, but they "report" the same in UOA metals). Also, you have to realize that statistical variation is always in play; it's hard (if not impossible) to nail down one single magnitude as "bad" or "good". This is more a topic of the shades of gray.
Extended OCIs are not just about one sole thing; they are a culmination of many concepts coming together in concert. When you understand how they work, and how to use them, they can be of great benefit. Knowing how to read UOAs (not just metals, but contamination and acid/base), as well as monitoring coolant, fuel, leaks, compression, noises, filtration PCs, etc, are all important to decsions points. And not the least of which you also have to understand statistical modeling to apply concepts of variance, to understand what is and is not "normal" for your application.
I have many friends that know I'm into lubes; it's a gear-head thing. But most of them do not have the desire/knowledge/paticence to understand the real world of lube maintenance programs. For them, I just tell them to OCI when the OEM says so. To pay for UOAs, and not understand their useful purpose, is a bigger waste than just dumping oil too soon.
You can either listen to the concert of knowledge, or hear the cacophony of rhetoric.
Your choice.
