Assume a Grp II+ or better oil. So let's say the engine oil hits TBN=0 - so, what's next? If the TBN gets to zero, then I'm assuming it was an acid balance that got it there - but with a grp II+ there are essentially no nitrates or sulfates in the oil to form acid - so is it all coming from combustion products? And I don't understand this TAN thing - acidic level of the oil. (??) Not sure how TBN and TAN coexist. And what's the ramifications of TBN=0? I read once (somewhere official I'm pretty sure) that TBN=0 doesn't necessarily mean any thing for engine components (but this is probably a matter of context). And even the stoutest oils often seem to drop like a rock from their lofty levels of TBN=12 to a TBN=3 and then level off. Now what's that about?