What is a TBN, and who uses it? In short, a TBN (total base number) measures the amount of active additive left in a sample of oil. The TBN is useful for people who want to extend their oil usage far beyond the normal range.

The TBN of a used oil can aid the user in determining how much reserve additive the oil has left to neutralize acids. The lower the TBN reading, the less active additive the oil has left.

Note: You do not need to send in a virgin sample for us to run a TBN on your oil. Some people like to send a virgin sample anyway, to find out exactly where the TBN of a specific oil starts out. The cost of a virgin oil sample plus TBN is the same as a regular oil sample plus TBN.

An oil's function is to lubricate, clean, and cool the engine. Additives are added to the oil to enhance those functions. The TBN will start out reading in the 6.0 to 14.0 range (depending on whether the oil is for gas or diesel engines). When you first start using the oil the TBN tends to drop sharply. Then it  levels out and drops more slowly after that. A low TBN test result, meaning very little additive is left, is down around 1.0 or lower.

Scientifically speaking, the TBN is one of two "neutralization number" tests run on oils. The TAN (total acid number), which is used for hydraulic oils, is the other. The TBN measures how much base (as in, a base vs. an acid) additive is in the oil to offset the deleterious effects of acids coming into the oil from combustion and other sources.

However, the TBN is not the only factor to consider when determining how long an oil can be used. If wear accumulations and insolubles in the oil build up and become abrasive, we would recommend changing out the oil, no matter how high the TBN reading.

Case Scenarios

Case #1
Joe Owner has a new engine and wants to determine how long he can keep a fill of oil in place. He sends in a sample of his virgin oil (with no miles on it) and a sample of his used oil (with 3,000 miles on it) for an analysis and a TBN test.

The virgin oil has a TBN reading of 12.0. His used oil has a TBN reading of 9.5, and his wear levels are fine. Joe decides to leave his current fill of oil in place, and resamples in another 3,000 miles. This time, the TBN reads 6.0, still an average amount, but his wear accumulations and insolubles have built up to a level that causes the oil to become abrasive. We recommend that Joe change out this oil.

Case #2
Jill Owner has a new engine and wants to determine how long she can keep a fill of oil in place. She sends in a sample of her virgin oil (with no miles on it) and a sample of her used oil (with 3,000 miles on it) for analysis and a TBN test.

The virgin oil has a TBN reading of 7.0. Her used oil has a TBN of reading of 5.0, and her wear levels are fine. Jill decides to leave this fill of oil in place, and she resamples in another 3,000 miles. This time, the TBN reads 4.5, and her wear levels are still low. Jill decides to leave her oil in place a bit longer, and she resamples in another 1,500 miles. This time, the TBN reads 2.5, and although wear accumulations are still normal, we recommend she change out the oil.

Do you need the TBN test in your maintenance routine? Only you can answer that question.  We offer a TBN test on any gasoline or diesel oil sample for an additional $10.