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Historically, the characterization of fresh and used diesel engine lubricants has been based on a limited number of analytical techniques. One of the most important analyses has always been the total base number (TBN) measurement. Although the TBN measurements are informative, easy, and quick, it can be misleading to base the judgment of an oil's performance solely on one criterion. This paper offers observations from a field test, showing that some detergent approaches gave unacceptable performance even though the TBNs were at an acceptable level. It is hypothesized that some detergents do not effectively neutralize all acidic species present in the lubricant, thereby reserving their own base while in fact the oil may no longer provide sufficient protection against bearing corrosion. This hypothesis is supported with bench and engine test data. It is recommended that, at a minimum, total acid number (TAN) measurements be included in any analysis.