Base number is still needed in diesel engine oils to neutralize acids that form. Also the metalorganic additives which contribute to alkalinity reserve in the form of base number also provides important detergency properties to the engine oil. These are most commonly represented by the calcium and magnesium containing additives. Even though the base number starts out lower in today’s engines oils base number retention is actually better in today’s API diesel engine oil classifications. There is still some level of sulfur present, which can create highly corrosive sulfuric acid. And due to exhaust gas recirculation NOX compounds react and form a weaker nitric acid back in the oil. But the general rule used in years past of 50% reduction no longer has any real relevancy and the base number can go to very low levels while in service and not be a great concern. The level of base alkalinity is adequate to combat the level of acid creation in today’s diesel engine oils, even when the base number gets to low levels during service.