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Combine the tools

Every oil has a starting acid number based on the amount and type of additives. As such, you will want to become familiar with TAN (total acid number or acid number) and TBN (total base number or base number).

TAN is the measure of the amount of acids, and is most commonly used with hydraulic fluids. It's important to know where it starts, so you will want to provide your lab with a new lube reference sample for each oil being tested. As the oil degrades, this number will increase as a result of oxidation, water, fuel or other contaminants in the oil.

TBN measures the alkalinity reserve of an oil, i.e., the ability of the oil to neutralize acids that form when diesel fuel mixes with air during the combustion process. This number will begin to decrease immediately.

"The theory with engine oil is to find the ideal point to change the oil," says Minges. "You can do that by running acid and base number tests."

For example, let's say you have a fluid with a starting point of 10 TBN and 2 TAN. As the base number depletes, the acid number increases. "Ideally, you want to change the oil when the two numbers meet," Minges advises. "If you get to a base number of 4 and an acid number of 4, that means the oil no longer has the ability to neutralize the amount of acids in the oil."