I can show you a bunch of Schaeffers that does this. Fact is, in some of the comparisions, Schaeffers retention rate, even though it starts out with a lower tbn, actually retains as well or more than some. In this case,
http://theoildrop.server101.com/cgi/ultimatebb.cgi?ubb=get_topic;f=3;t=000053
This engine is very easy on oil but notice down near the bottom of this thread, there is also a comparision of mobil in a gas engine. Take note of the tbn retention vers mileage in that comparison. That particular engine is much harder on oils and the retention rate much harder to maintain. We have other gas engines that don't even come close to stressing the oil's tbn as much as that one did.
Point is, TBN factor is not a good way to determine a quality oil. Even though mobils didn't fair.
Mobil started with a tbn of 12, Schaeffers 8.3
So, Mobil, being a full synth, looked like a better choice over the Schaeffers 8.3 and it being a blend.
But as you do the analysis, you can see that even though mobil started out as a 12 tbn and inside of 4,300 miles was down to 5.6, and Schaeffers started out at 8.3 and dropped to a 3.6 with 9,000 miles, suggests mobil may not hold up as long in comparision to Schaeffers Blend at near half the cost.
See how the TBN #'s used as a guide to quality can really mis lead you?