I'm dying to get more info on how TBN is tested. My own sample showed TBN above 11 for the first 2000 miles and above 8 for the first 3000.
I suspect testing TBN may not be an exact science, with variations from one lab to another.
first there are two different systems for testing TBN: The most common is ASTM d 2896 (if my memory serves me); then another that I don't remember.
Second, all specs are "Typical" there is a sizable variance. A given oil might say 3300 ppm of calcium on the spec, when their acturl production range is 3000 to 3800. Since TBN is related to Calcium (among others), this will vary that spec. I've analized some 10 TBN oils that were 14 and some that were 7.
As far as what is low and high, typically gasoline oils are 5.5 to 8.5 from what I've seen; and Diesel oils (or "universal oils" from 8.0 to 12.5.
Note at the bottom of all spec sheets is a disclaimer that this is only representative of a "typical batch", not the specific lot.