If you don’t know an oils base number your sump could be a sulphuric acid bath!

So what’s an oils base number got to do with Sulphuric acid?

You’ve probably heard the commentary in the media about the Sulphur content in diesel fuels – did you know that high Sulphur content is a major factor in reducing oil life in diesel vehicles? And the lower the oils base number, the greater the risk of acid build up.

How do we define an oils base number.

The oils base number (BN) is a property
that is more associated with engine oils rather than industrial oils. It can be defined as the oil’s ability to neutralize acids that are produced during use.

The higher the base number in the engine oil, the more acid it will be able to neutralize during use. New engine oils usually have a range of 5 to 15 BN.

As oil is used in service, it becomes contaminated with acids, causing the base number to drop over time. By using oil analysis for your engine oil, you will be able to track the BN of your oil and determine how much life is remaining.



Once the base number drops below 3, this is considered too low and should trigger an oil change for your engine.

Why would an engine oils base number drop?

The most common reasons for a drop in the base number are related to low-quality fuel and oil oxidation.

By way of example: In a Diesel engine using diesel with say, a 500PPM Sulphur content, sulphuric acid will be produced during the combustion process, which attacks the oil and causes a drop in the base number.

Oil oxidation as a result of the engine overheating or an attempt to extend the oil drain interval is another reason you may see a drop in the BN.

As oil is used in service, acidic components are generated and build up in the lubricant, with the end result being an increase in the acid number.

A high acid number represents the potential for corrosion, rust and oxidation. It can also be a signal to perform an oil change. Again, by using oil analysis, you will be able to track the AN of your oil and schedule oil changes.

The acid number (AN) is a property that is generally more associated with industrial oils than engine oils. It is the amount of acid and acid-like substances in the oil. As mentioned previously, oil oxidation is one of the main producers of acid.

You’ll also need to set a critical limit for when the acid number reaches a certain number in order to schedule an oil change.

This critical limit will be dependent on the type of oil being used. Typically, for R&O or light-duty oils, a maximum acid number limit of 2 is appropriate. For anti-wear and extreme pressure (EP) oils, an AN limit of 4 is a good starting point.

To summarise: An oils base number is an indication of its resistance to acid degradation.

We may not be able to control the Sulphur content in fuel, but Habot Synthetic Lubricants are of the highest quality, so feel free to contact our professional staff for advice on your next oil change.