Normally you want higher TBN, as long as the Sulfated Ash comes in below 1.5. But it is important that base stock not be volatile or it will flow through to the catlitic converter. SA is a factor of the additives. Calcium, Mag,zinc, phos, moly, etc. Normally the bulk of the TBN comes from the Calcium and Mag (detergent/dispersant) but there are also non-metalic dispersants that can be used (such as in two cycle oils) that are also blended into the regular oils to keep the SA low but the Detergency/dispercancy/TBN high. They are expensive. Some oils start out with very high Calcium/Mag levels and drop off like a cliff. Others maintain 80 to 90% of it over their usefull life. (I've seen Delo drop from 3200 ppm calcium to 3000 in 20,000 km, where Lubrax went from 3600 to 2000 in 20,000 km, while the Zinc in the Delo went from 1500 to 1300 and in the Lubrax went from 1300 to 850 with a similar drop in phos.)Quality of base oil and quality of additives are more critical than amount once they are above acceptable limits.