Where doz the TBN # fit in to all of this, or is it just part of the additive package that addresses alkalinity and has no bearing on viscosity?
Depends on who is doing the marketing on TBN. It was a big deal in the early 2000's but that has died down a bunch.
TBN, like most other stuff regarding oil has a purpose. IMHO, it's about the entire package and how well it fits your requirements.
For example, I never ever use a true sytthetic oil with alcohol based fuels.
And, I don't count group III oils as synthetic, even though the US government, through the Supreme Court has said that I should.
