What do Acid Number (AN / TAN) and Base Number (BN / TBN) mean on a lube oil sample analysis report?

This article will answer the following questions:

  • What is a Total Acid Number (TAN) and what is a Total base number (TBN)?
  • When to change engine oil and what indicates an oil is spent?

TAN TBN What do Acid Number (AN / TAN) and Base Number (BN / TBN) mean on a lube oil sample analysis report?

Acid Number – This is the concentration of acid in the fluid. The new oil value may not be zero since some oil additives react similar to acids with the reagents used for testing. Hence it is important to monitor the trend in change of TAN from a baseline sample or new oil reference. In engine applications this can be used in conjunction with a Base number value with the crossover of the two values a common method of deciding when to change oil. However, always be guided by your lubricant provider and OEM recommendations.

Base Number – This is the amount of alkali additive remaining in the engine lubricant. This is used in conjunction with an AN value and oil changes are typically either on a drop of 50% from a baseline sample or when the value becomes less than the AN. Again, always be guided by your lubricant provider and OEM recommendations.

Common CausesPotential ResultPotential Advice
High Acid NumberAnd/orLow Base Number
  • Overheating
  • Excessively extended drain interval
  • Wrong lubricant
  • High sulphur fuel
  • Decreased BN
  • Corrosion
  • Oil Thickening and Sludging
  • Increased wear
  • Filter blocking
  • Component Seizure
  • Evaluate oil drain interval.
  • Establish new baseline BN / AN values.
  • Change oil
  • Submit fuel sample for analysis
  • Submit coolant for analysis
High Base Number
  • Wrong oil added / topped up.
  • Cylinder oil contamination.
  • High Ash (SAPS) production
  • Wasted cylinder oil.
  • Reduce Cylinder oil fuel rate
  • Confirm correct lubricant in use.