An oil analysis report usually has twenty lines. Three of them decide whether you change the oil, shorten the interval, or investigate the machine — and two of those three are routinely misread by buyers who are otherwise competent.
TBN is not "remaining life"
Total base number measures alkaline reserve — the oil's capacity to neutralise acidic combustion by-products. A new CI-4 15W-40 might start at 10 mg KOH/g and reach 4 after 400 hours. That does not mean 60 % of the oil's life is gone. TBN depletion slows as the oil ages, and the additive system is only one of several life limits.
A more useful reading is the rate of decline between two samples. If TBN drops from 10 to 7 in 200 hours, and from 7 to 6 in the next 200, the curve is flattening and the oil still has reserve. If it drops 10 → 6 → 2, condemn the oil regardless of hours.
Condemnation limits commonly used: TBN below 2 mg KOH/g, or below 25 % of the fresh oil value, whichever is higher. Always set your own limit against the fresh oil certificate for the batch you are actually using.
TAN rising faster than TBN falls is the real warning
Total acid number climbs from oxidation and from acidic combustion products. A slow, steady rise is normal. A jump of more than 1.0 mg KOH/g between samples, or a crossover where TAN exceeds TBN, means the oil is oxidising faster than its reserve can absorb — usually heat, extended drain interval, or a coolant leak. That is the point to inspect, not to top up.
Water: 0.1 % is not "a little"
Water content below 0.1 % is unremarkable. Between 0.1 % and 0.3 % free water starts to accumulate and emulsify, particularly in hydraulic systems where demulsibility is the whole point. Above 0.3 %, you are looking at rust, additive precipitation and, in engine oil, a serious risk of oil starvation at the bearings.
In tropical operations the usual source is not a blown gasket — it is condensation from daily heating and cooling of a reservoir that is never fully warmed, or a breather that pulls in humid air. A desiccant breather often fixes it for the price of a drum of oil.
The three-line summary we send to customers
| Parameter | Watch for | Action |
|---|---|---|
| TBN | Below 2, or falling > 50 % in one interval | Change oil, check fuel sulphur |
| TAN | Rise > 1.0 between samples, or TAN > TBN | Investigate heat / coolant ingress |
| Water | > 0.1 % (hydraulic) or > 0.3 % (engine) | Find the source; filter or change |
Two more worth reading once the three above are under control: viscosity change beyond ±10 % (usually a wrong top-up or fuel dilution) and wear metals trending rather than absolute values. A single iron reading of 60 ppm means little; a rise from 20 to 60 over three samples means something is changing.
If you would like a template monitoring schedule for a genset fleet or an excavator fleet, ask our technical team — we send it as a spreadsheet, free.