TAN measures the amount of acid present in the lubricant, which is independent of the remaining TBN or Total Base Number. As acids form, they get neutralized by the over based detergents in motor oil and get sequestered into detergent micelles. But they are still there and contribute to the level of oxidation and eventual viscosity increase. TAN is usually used for gear lube or hydraulic fluid analysis, where the lubricant typically does not contain a robust over based detergent package. TBN is a more useful metric for used motor oil since it indicates the capacity of the lubricant to continue neutralizing acids.