JimBrewer -
TBN and TAN are effected by three things:
1) use of lube
2) storage of lube
3) inputs of causation (acid inducement via fuel, lube, etc)
NOTE: I am predicating this conversation on equipment that is in good mechanical shape, and fuels that are not compromised/contaminated. If these things are not true, then the following information I discuss is moot.
In short, you can effect the relationship by having high mileage (or equivilant in hours such as stationary or off road equipment), or by letting a piece of equipment sit idle (dormant). Fuel (especially diesel) used to be an issue for Sulphur, as well all know. But even that is now GREATLY reduced as a concern.
I have both types of "use" going on at my home. I have cars that get driven a lot; I have equipment that sits for long periods of time.
This experiement of mine (along with the data from 2010_FX4) have shown to me that TBN/TAN is simply not an issue when it comes to vehicles in use, at least in a manner that would be "normal" or even a bit past normal. I ran 15k miles on ST dino lube, my TAN crossed over TBN multiple times, and yet wear is not effected greatly, if any. It cost me an extra $20 each time I got these tests. Before I had the data, it was questionable, but I had a hypothesis of interaction. I have now proved it to be so. 2010_FX4's data echo's mine. He has seen TAN cross over, and yet nothing horrid happened, and if we had never paid for the testing, none of us would have even been the wiser. So, I see zero reason to ever pay for TBN/TAN again in high-use, low storage equipment when OCIs are at or a bit longer than "normal".
I will include a big caveat here, that we all should pay attention to. This issue ceratinly will have a point where TAN does indeed become too big to ignore, and likely will have some effect on wear by eating away at parts. But my point is that we can see that up to 15k miles, that issue has not revealed itself yet. If someone would like to push the envelope further, then feel free to go for it. But at this point, I'm more than comfortable knowing that 15k miles isn't a problem; TAN will not degrade TBN to a point of massive failure. I wil never again pay for TBN/TAN unless I have a specific cause for concern in this regard.
As for the other topic, that of storage, I am still paying for TBN/TAN. I have multiple piece of equipment that see infrequent use:
Duramax truck, Kabota tractor, Scag gas mower, classic 1966 Mustang, various outdoor power equipment (thes all get 10w-30 HDEO) .....
I seek the data; I need to know how that relationship plays out in my barn. So I am still getting TBN/TAN for my Duramax, which is driven infrequently. But so far, it seems fine. And after three or four years on one OCI, if the data supports a solid relationship, I'll probably not pay for those ever again either. But only time will tell.
I may have a hunch on something, but I actually go out and test my hypothesis, and await the data before actually making a determination. I don't listen to rhetoric or mythology; I test and conclude only after data speaks. What the data tells me is that when in use, the TBN/TAN is not an issue up to 15k miles. The data for long term storage is pending, but the "ongoing" ramp of data is showing it's also favorable. But a conclusion at this point would be premature for that scenario. So, I continue to pay for TBN/TAN until I prove to myself it's moot in low use equipment, as I have with high-use equipment.