The wear metals are commendable. The contamination is low. There's no reason not to extend the OCI.
As for TBN/TAN, I continue to have the opinion that most other people's opinions are ill-informed. The inversion (when TAN crosses over TBN) of the base/acid relationship is NOT, in and of itself, a reason to change oil. When the "flip" occurs, it's a reason to perhaps monitor the fluid closer, a bit more often. There is not one shred of historical proof that as soon as the crossover occurs, horrible things happen. Way back in the day, when diesel fuels were high-sulphur and all engines had open crankcase vents (allowing a lot of moisture in upon cool down), acid build up could happen fairly quickly; those days are LONG gone. The crossover of TBN/TAN is a very good reason to start paying a bit closer attention to your wear metals, because at some point, the acid may become an issue. But it's NOT a foregone conclusion that crossover is going to nearly immediately destroy an engine. Crossover is a tell-tale sign that you should have a heightened sense of vigilance, not an OCD driven rush to the tool box for the wrenches.
The time to change oil is unique to each person; we all have a different threshold. Some go by time and/or distance; what "feels right" to them. That's OK. Other's choose a deeper analytical approach; UOAs. But even UOAs can be either used as a toy, or a tool. A UOA is a toy when you pay for data and then ignore the data and make conclusions based on partial truths or illogical condemnation points. A UOA is a tool when you understand the entire relationship between the following:
- individual equipment history (what has your engine/tranny/diff/bearings been exposed to in terms of service factor? micro-data)
- series equipment history (what does the engine/tranny/diff/bearings typically exhibit when 100k other people also own them? macro-data)
- lubricant construction (base stocks, additives, vis, FP, etc)
- what environmental elements are present in typical operation?
- do you understand the concept of the TCB and how it affects wear?
- do you understand that UOAs are not perfect, and to use the UOA correctly, you have to understand its limitations?
- do you understand that UOAs are by far and away, the cheapest and easiest way to predict wear, as opposed to other methods?
- do you also pay attention to other clues such as noises, and visual indicators like fluid levels?
- do you use the points above to set condemnation points which are realistic for your overall maintenance plan? (wear rates, wear totals, contamination, etc)
I have over 20k UOAs in my database, and with rare exception, the base/acid crossover has historically shown absolutely no correlation to an increase in wear metal rates in most circumstances. And without correlation, there can be no causation.
'Nuff said.
