Neutralization Number

AN: (Formerly TAN) Acid Number. ASTM D-974 and D-664 Oxidation by-products tend to be acidic. This test measures acidic material in lubricants. Usually (but not always) indicates potential for corrosive wear. The rule of thumb for a condemning limit is twice the AN of the new oil or an increase of one whole number. This test is usually for non-engine lubricants.

BN: (Formerly TBN) Base Number. ASTM D-4739 and D-2896 This test measures the alkaline reserve of the lubricant (ability to neutralize acid). The rule of thumb for a condemning limit is ½ the BN of the new oil. The test is usually for diesel engine lubricants.
ASTM D-2896 is used for new oils.