Question: 

What are TAN and TBN?

Answer: 

The most basic explanation of these specifications is that TAN means total acid number and TBN mean total base numbers. Technically oriented people will want to give you a textbook definition of these specifications, but here is the relevance of these specifications. If you are taking advantage of an oil analysis program, testing for the TAN number can tell you how much buildup of corrosive acids is occurring in the engine oil and the level of oil oxidation, or what is happening to the oil as a result of sustained heat stress on the oil that will result in oil thickening and oil breakdown over time. Think of TBN as a number that shows the level of reserve anti-acid or acid neutralization remaining in the used oil. Both of the specifications are identified as a number on used oil analysis reports. ADVERTISEMENT

TAN will typically increase with time on the oil and TBN will decrease over time as the acid neutralization oil is being consumed during service. Oil analysis laboratories will provide condemning limits on both TAN and TBN, which will provide one aspect to oil condition. Traditionally, one way of evaluating oil condition is to know when the used oil TBN number drops to the level of the increasing level of the TAN number, which is often interpreted as the point where the oil can no longer sustain adequate corrosive wear protection. It is recommended you  refer to your engine manufacturer's service bulletin regarding the acceptable ranges of used oil, and if you are using oil analysis, ask both your oil analysis and lubricant providers about their recommendations.