Lube Condition Indicators
Generally, a new lube starts with the highest TBN it will exhibit. From that point, TBN will tend to decrease as acids consume its alkaline properties. The addition of fresh makeup lube, however, helps forestall the decline in TBN, often allowing extension of lube drains in properly maintained engines. This will naturally vary from unit to unit, dependant upon individual conditions.
Typical motor lubes have TBNs of 5-7, and highly compounded oils may have TBNs of 12-15. Marine oils for engines using high sulfur fuel may have TBNs of 35-40, while upper cylinder lubricants in those same engines may exceed 70 TBN.
NOTE: TBN alone is not a relative indicator of lube quality, only its ability to neutralize corrosive acids. A higher TBN clearly demonstrates better capability to counteract acids, but does not mean the higher TBN oil is better than, or even equal to, a lower TBN lube with respect to other important properties.
It should also be noted that most motor lubes can have both a TAN and TBN in the new and used state. If one regards these terms as two distinctly different properties, there is no difficulty in explaining their simultaneous presence. Ideally one would perform analyses for both terms in the inspection of motor lubes, but TBN is by far the most important property of the two for motor lubes. TAN is usually acceptable when TBN is sufficient.
TAN is almost exclusively used for non-engine systems because there is no combustion process taking place, and because many non alkaline lubes are in use in such systems, making the analysis for TBN pointless TAN also applies for large stationary 2-cycle gas engines using "mineral oil" type lube.
A Word About pH: pH and TAN are oftentimes confused. The concepts are quite involved but succinctly, pH is-relative acid strength while TAN is total amount with no regard for strength. Weak acids, such as lube-oxidation products, do not significantly affect pH.
