I always thought the base number was the measure of how the oil was setup to handle acidity, for example the base number of new Chevron Supreme is 7.1 and the Delo400 is 11.3. (Delo being made for diesels would compensate for extra sulfur in the fuel...)
Ph is neutral at 7.0 and VERY acid down near 1, alkaline (basic) near 14. When the base number degrades down to a low number, it is time to change oil because now the number (ph) is getting too acidic (and acidic means that the metals start seeing corrosion..). That is the main reason that I consider the 3k oil change a good thing,,,,filters can't take out acidity. Newer oils (SL) probably handle acids better than older ones did though, probably do last longer, but I change at 3k anyway.
As for the answer to your question, which componant compensates for the acidity? Someone else on here will have to answer that one. From a chemistry point of view, whichever additive it is that compensates for acidity would be called a "buffer". From what I have read elsewhere, the detergent would be responsible for the most part.
[ July 20, 2002, 07:20 PM: Message edited by: ZR2RANDO ]