My guess is a number of adds provide TBN, including calcium and zinc. Also, I though I read that an ester base oil provides a TBN effect although not measured in the TBN test, which might explain why Redline TBN is not as high as some would expect. OTOH, maybe I don't know what I am talking about (except calcium--a no brainer TBN constituent).
Hey, I bet a search of the Question of the Day and/or Interesting Articles forums would answer the TBN question.
Seems like you guys are on the right path, but you may want to search the "Question of the day" forum.
Basic (and di/tribasic) compounds (yes Ca, Mg) as well as amines (alkanolamines, etc) all provide acid nuetralizing properties (taking H+), therefore in a lab test will give a (higher) TBN....
But there is more to keeping an oil in use longer than a relatively high TBN.