The Total Base Number (TBN) indicates the basic(alkaline) nature of an lubricating oil used in engines. Higher the TBN, more basic is the oil. Such an oil can withstand highly acidic impurities.
TBN is measured in mgKOH/gram. This indicates the milligrams of Potassium Hydroxide that each gram of the oil is equal to. Oils used in diesel engines will require a higher TBN as they have to withstand aggressive acidic residues. Gasoline engine oils have a lower TBN.
As the engine runs, the TBN of the oil gradually falls due to reaction with the combustion residues. The TBN has to be periodically checked. A sample of the oil is sent to laboratories for checks.