Testing a lubricant's base number measures its alkaline reserve - or its ability to neutralize degradation acids. Although viscosity is most important, base number determines an oil's suitability for continued use and is critical to safely extending drain intervals. A decrease in base number is typically accompanied by an increase in oxidation, nitration and viscosity.

Sulfur from fuel and nitrogen from the air - when combined with water - form sulfuric and nitric acids that will attack engine metals causing wear that can eventually result in engine failure. To combat combustion byproduct acids, diesel engine oils contain acid neutralizing detergent additives - the most common being magnesium and calcium.

New diesel engine oils typically have a starting base number between 8 and 12. It is generally recommended that diesel engine oil be changed when the starting base number is 50% of the new oil. For example, an engine oil with a starting base number of 10 should be changed if the base number test result is less than 5.0mg KOH/g.