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Old 05-08-2012, 01:51 PM
m6pwr m6pwr is offline
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Location: San Diego, CA
Join Date: Jan 2008
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Mein Auto: '14 335i M Sport
I have to reiterate, I'm no expert, but I'll try to explain what I know. The short answer is no - - the TBN should never be allowed to reach 0 and the TAN starts to build well before the TBN ever reaches 0.

TAN is the concentration of acid products in the oil. It is not a measure of the acidity of the oil as if it were an acid bath. Nevertheless, if TAN gets high enough, corrosive wear can take place. With modern oils, fresh, unused oil can have an acid number/TAN right out of the bottle because it contains some acid products - - some of the additives are acidic, particularly magnesium which is being used increasingly as an antacid in place of calcium (which lays down more unwanted deposits). In addition to these acid products in the oil, the acid number begins to build as soon as the oil is in use. Oxidation and nitration will raise the TAN as the oil ages.

TBN or total base number is a measure of the reserve alkalinity of the oil - - its ability to neutralize or take up acidic components in the oil. The TBN starts to drop as soon as the oil is in use.

Now comes the question of how do you use these numbers (and other key indicators in the used oil analysis) to figure out when to change the oil (the so-called condemnation point). By coincidence there's an article on just this topic in the May issue of Tribology and Lubrication Technology magazine. The bottom line: there's no agreement at all on this. There are three "players" in this discussion: the oil analysis labs, the lube mfrs, and the engine mfrs. The labs will give you their cp's when they do your used oil analysis (uoa). The labs don't all agree on the same cp's. The engine mfrs and the lube mfrs do a lot of collaborative testing/research to come up with their cp's for a specific engine but these are not widely published - - they are in effect hidden in the oem's prescribed oil change interval and the oil life monitors/software, if so equipped. The oem's cp's are probably the best.

There's an interesting discussion of this topic here - Doug Hillary (one of the commenters in the thread) is a very experienced euro-based lubrication engineer who has decades of experience with Castrol and Mobil. He knows some of the oem's cp's. The cp Hillary uses for TAN/TBN "see saw" as he calls it is TAN = 6, TBN = 2.
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