I think TBN starts at 12.9.Mobil is obviously great oil but no excuses accepted, this is a poor showing for M1 0w40 plain and simple. I'd bet either Castrol 0w40 or 5w40 would have at least a higher retained viscosity than this.
TBN is fantastic but it's due to the dump truck load of calcium Mobil uses in this.
It will be interesting to see how the bargain priced QS 5w40 holds up compared to this.