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Does oil really need to be changed at 50% TBN reduction?

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In the stationary gas engine industry there are many ‘urban legends’ and ‘rules of thumb’ about lubricants. Often these ‘rules’ are very important because they steer the direction of gas engine oil development but they also contribute to misinterpreting the performance of stationary gas engine oils. A few examples of these urban legends are:

  • replenish at 50% TBN

  • Group II is better than Group I

  • more ash, more deposits

Challenging these accepted rules is very important to make the next step towards better and more innovative stationary gas engine oils. I have written a series of articles about these urban legends and will publish these on LinkedIn in the coming weeks. The first article is about the 50% TBN rule.


The 50% Total Base Number rule is probably the most unquestioned rule of thumb in the industry. To be fair, for most traditional stationary gas engine oils (SGEO) it works quite well. But, better understanding of the rule is needed to make the next step towards better oils.

What is TBN and how is it used?

The TBN is a measure of the alkaline concentration in a lubricant. This value is used when describing the detergent capacity (base reserve) of the oil; in other words, a higher TBN indicates greater acid neutralising capacity.

The most common test in the industry is the ASTM D2896 test method. It provides an accurate indicator of the TBN and the results can be compared to the unused oils’ TBN. The 50% TBN limit means that the stationary gas engine oil should be changed when the used oil reaches a TBN value which is 50% of the fresh oil value. For example, when the fresh oil TBN is 6 mg KOH/g the used oil should be replenished when its TBN value reaches 3 mg KOH/g. Below the 50% value there is insufficient acid neutralising reserve in the oil, which leads to oil thickening, increased oxidation and fast lubricant breakdown.

Many Original Equipment Manufacturers (OEMs) apply the 50% TBN rule. These OEM used oil limits are very important in the stationary gas engine segment because warranties and service contracts are fixed to them. As a result, they determine the drain interval of oils, and therefore the perception of quality.

Why should we rethink the rule?

To understand why the industry needs to rethink the 50% TBN rule it is important to understand first the ASTM D2896 test. This is a titration test in which you neutralise the oil with a strong acid, using the very strong perchloric acid in the titration process. The amount of acid needed in the test determines the TBN value. Perchloric acid is so strong that the test measures all basic constituents., both weak and strong bases.

With traditional stationary gas engine oils this works well, because the base number equals the amount of overbased detergents, which is a good indicator of the acid neutralising reserve.

Modern clean technology gas engine oils are based on a more complex additive chemistry compared to traditional SGEOs. Apart from only strong base material, like overbased detergents, these products contain a variety of weak base materials. Examples of weak base components are some types of anti-oxidants and metal deactivators. See also the table below.

Components ASH
Overbased detergent (neutralise acids) Y
Dispersant (solubilise soot) Y/N
Neutral detergent (keep surface clean) Y
Anti-oxidant N
Metal deactivator (deactivate catalyst) N

This means that in modern clean technology SGEOs the TBN value is no longer just a measure for the acid neutralising capacity; it is a combination of all the additives which contribute to the TBN number.

But what about the longer-term impact?

The issue is that these other weak base components can be used up in practice. For example, anti-oxidant reacts with oxygen and is consumed; as a result, the TBN decreases. To prevent this TBN drop, some lube oil companies remove these components from the lubricant formulation. The perception is extended drain performance, which is confirmed by laboratory routine analysis testing, but the long-term effect is dirty engines, more oil deposits, increased downtime and higher maintenance costs.


The perception is extended drain performance, which is confirmed by laboratory routine analysis testing, but the long-term effect is dirty engines, more oil deposits, increased downtime and higher maintenance costs.

The black dotted line is the traditional SGEO; the black solid line is the high-performance clean technology SGEO; the grey area is the decrease of the weak base components, like anti-oxidant, which do not contribute to the acid neutralizing capacity. Removing the important other components improves TBN retention, but has negative side-effects.

Think again about how oil quality is determined

The 50% TBN limit and the perception that SGEO quality is mainly determined by routine analysis reports, not by long-term effects like cleaner engines, set the conditions for lube oil development. This is a consequence of how limit values are set and how oil quality is determined. Modern clean technology gas engine oils are often replenished too soon. This false perception of poor quality prevents their growth in the market and users do not benefit from the clean technology performance.

Farewell to the legend

It is time to say goodbye to the 50% TBN urban legend. Performance of the engine oil should be the leading consideration and the SGEO should be replenished at the right limit value; this will likely mean specific limits for some oils instead of generic limits for all oils.

Source: expert.q8oils.com