I'll say it, and I'll stand behind it ...
TBN doesn't matter in most UOAs because most UOAs don't represent extended OCIs.
TBN doesn't matter when the wear rates aren't affected.
TBN doesn't matter nearly as much today because fuels (both gas and diesel) have much, much lower sulphur content
All that said, knowing TBN isn't worthless, but it needs to be understood in context. TBN and TAN can be indicators of the POTENTIAL for change due to corrosive concerns. TBN and/or TAN are not reasons to condemn a fluid in and of themselves. What one should be looking for is both the rate of change of TBN and TAN as well as overall magnitudes, and the possible resulting onset of uptick in wear metals due to acid etching of the surfaces. If TBN and/or TAN are changing, and the wear metals go unaffected, then all is well for the time being. A strong TBN value in the UOA indicates that it's likely that corrosion isn't going to take hold. However, a low TBN value doesn't mean wear is automatically afoot. It only means a possible wear shift is more likely, and that should trigger a closer monitoring effort (such as shortening the UOA intervals to see if wear trends increase; the sooner you can catch a trend shift the better).
In wwillson's UOAs, he's running very long OCIs. TBN and TAN are helpful as predictors for potential changes in wear.
In most UOAs run by Joe Average, their short-to-moderate OCI durations don't give any reason to pay attention to TBN/TAN because there's no evidence that the data has any correlation to wear rates in those conditions. And without correlation, there can be no causation.
