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According to Wikipedia,

Total Base Number (TBN) is a measurement of basicity that is expressed in terms of klk 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 6–8 mg KOH/g in modern lubricants, 7–10 mg KOH/g for general internal combustion engine use and 10–15 mg KOH/g for diesel engine operations. TBN is typically higher for marine grade lubricants, approximately 15-80 mg KOH/g, as the higher TBN values are designed to increase the operating period under harsh operating conditions, before the lubricant requires replacement.

But, can be there be a too high TBN?

For example, what could happen if a 10-15 TBN oil was used to lubricate a regular non-diesel car/motorcycle engine, instead of the standard 7-10 tbn one?

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You might want to read the article here:

TBN Discussion

But the key point on too high a TBN number is:

So should we all use a High TBN oil? There are advantages, but there are also disadvantages of using a High based TBN oil (known as High Overbased Sulphonates) it's not just because the price is much higher. High TBN oils can produce high ash content that can impair engine efficiency and cause loss of power, this will lead to excessive deposit build-up on pistons and valves. High ash is caused by non-combustible residue of an oils detergent additives, these additives contain derivatives such as barium, calcium and magnesium. These deposits can lead to the following devastating results:

> Ash deposit build-up on pistons and valves 
> Loss of oil stability
> Polishing of cylinder line bores 
> Valve Guttering 
> Loss of power
> Increased oil consumption 
> Increased fuel consumption

So the point is, I believe, use the proper oil specified by the engine's manufacturer. If the TBN is a little high, as in your hypothetical case, it's not too much of a concern. If it's WAY high, then you may have problems.

  • Looks like there are a lot of unsubstantiated claims in that article! First of all, how does too high TBN cause "buildup" of ash? Wouldn't it instead encourage the ash to be more solubilized in the oil solution, to be later then just properly filtered out via the oil filter mechanism (consequently)? Second of all, why would magnesium, calcium, barium, etc... even have an ash-promoting effect in the first place? Is there some kind of ash-promoting chemical catalysis going on? Does acid neutralization create new additional ash for an engine to deal with? – ManRow Nov 17 at 13:06
  • Perhaps the issue of ash is when oil is part of the combustion process... @ManRow – Solar Mike Nov 17 at 13:11
  • @ManRow Some oil always ends up in the combustion chamber since to valve seal or piston ring is 100% effective. Most of the time this is of little consequence as long as it's a small enough amount not to interfere with the combustion process. Other elements, such as those listed in the article, don't combust and tend to stick to the piston and cause ash buildup. – jwh20 Nov 17 at 13:29
  • Hmm, it seems I'm asking for two different things in this question. One is what the dangers of too high TBN are, and the second is what is the underlying chemical explanation for those effects (interference with proper ash solubilization and filtration, possible promotion of extra ash formation, etc...). The link you provided for the first question just makes various claims about the dangers of high TBN, and it seems we must trust those on a matter of faith since no evidence nor any other substantiation (or explanation why) is provided. – ManRow Nov 17 at 14:17
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    The stuff called "ash" is insoluble by definition. If it was soluble, it wouldn't be "ash". Basically, if you are putting metals like magnesium, calcium, barium, etc in the combustion chamber, you are likely to be manufacturing some sort of ceramic particles (which are physically hard and chemically unreactive), and then hoping the oil gets rid of them before they do any damage. – alephzero Nov 17 at 15:28

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