TBN (Total base number) indicates the oil's ability to neutralize acids formed during combustion or oxidation. A high TBN means good protection against acid formation, while a low TBN suggests that the oil is beginning to lose its protective properties.
This analysis is also carried out by titration, but here an acid is used to neutralize basic components of the oil.
The result helps the customer assess whether the oil still provides adequate protection, or whether it is time to replace it. Especially relevant in engines running on sulfur-rich fuel.