Historically, Total Base Number (TBN) has been a key indicator of remaining useful oil life in heavy duty engine oils. While acid neutralization is still an important function of engine oil, changes in engine design and the move to Ultra Low Sulfur Diesel (ULSD) fuels have decreased the amount of acids produced in the engine. This also results in generally weaker acids being produced. This article explains the differences in ASTM test methods for TBN, industry changes leading to the current categories and the change in how TBN values may be interpreted with the new oils.
By Phillips 66 Lubricants