Problem is that no one can define a good generic oil analysis result? Amsoil OAI, could say that after 15,000 miles a TBN of 4 is fine keep going, many on this board would say oil is shot. If we went to 25,000 on the same oil and the TBN was 2 did our engine suffer premature wear by going the extra 10,000, did wear increase at a higher rate in the last 10,000 or wsa it at the same rate. Thus, 15 ppm of iron at 15,000 miles OAI would probably say that 25 ppm of iron is perfectly fine at 25,000. Wear rate is the same.
OAI I think is one of the few that looks at extended drains and the rates of wear as opposed to Analysts Inc (more of a percentage change with them) and Blackstone (which I think is the most conservative) Blackstone can and does use dino and synthetic results when calculating their universal averages for wear.
There are so many variable. The key is will the engine die sooner if we push that oil to 25,000 with what we may consider high wear numbers and a low TBN or will it be around as long as if we had changed the oil at 15,000.
Thus, we cannot prove Amosil wrong (although I would not push any of their oils beyond 12,000 without analysis and only if I drove 90% highway miles) nor can we prove ourselves correct and say their oil will not hold up that long. The two or multiple engine comparisons do not exist to make a conclusion IMO.
As the LS1 test is showing with the Mobil 1 the TBN and wear numbers can actually bottom out at a reasonalbe number and the TBN rise slightly over time. We just do not know.