Know the Proper Base Number of Your Oil

Generally speaking, the higher an oil’s total base number (TBN), the better its ability to neutralize contaminants such as combustion by-products and acidic materials. TBN is a measure of (alkaline) additives in the oil. Higher TBN oils are able to neutralize a greater amount of acidic materials, which results in improved protection against corrosive reactions and longer oil life.

TBN levels are targeted for the intended application. For example, gasoline motor oils typically display lower TBN numbers, while diesel oils must manage the high contaminant-loading from soot and sulfur and typically have a higher TBN.

TBN levels decrease as the oil remains in service. When the level reaches a point where it can no longer protect against corrosion, the oil must be changed.

Oils that are formulated specifically for extended drain intervals typically display elevated TBN to ensure proper corrosion protection for the duration of the extended interval.

The 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.

The most common reasons for a drop in the base number are related to low-quality fuel and oil oxidation. During combustion, a low-quality fuel with high sulfur content can produce sulfuric acid, 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.

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.

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.

You also will 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.

Total Base Number (TBN) is a measurement of basicity that is expressed in terms of the equivalent number of milligrams of potassium hydroxide per gram of oil sample (mg KOH/g). TBN is an important measurement in petroleum products, and the value varies depending on its application. TBN generally ranges from 6–80 mg KOH/g in modern lubricants, 7–10 mg KOH/g for general automotive use and 10–15 mg KOH/g for diesel operations. TBN is typically higher for marine grade lubricants, approximately 15-80mgKOH/g, as the higher TBN values are designed to increase the operating period under harsh operating conditions, before the lubricant requires replacement. An oil formulation contains basic additives, designed to react with and neutralise acids , preventing damage to engine parts, including corrosion of metal surfaces and damage to engine parts.