[JFP in PA]

The problem with the TBN values lies in the fact that once an oil is past the 50% mark, it is well on its way out and should be changed (a 50% drop from baseline has long been considered by many as a "yardstick" for when to change the oil). So, yes, there is usually a pretty quick fall off, then a more tapered decrease; but the 50% yardstick still holds regardless of the drop off rate, plus other brand oils do not show this level of drop off is so few miles in the same vehicles. And, as others have posted, we are not the only ones seeing this level of performance. Plus, not everyone that keeps data on oil performance makes it available to a Google search…..

As for an “X”W-40 oil shearing down to a viscosity equivalent to a 20 weight oil not being harmful, sorry, but I cannot agree with you. As Toyota recently discovered with their recommended use of 5W-20 oil in their large V8 pickups’ to get a better CAFÉ rating, premature engine replacements resulted in a mandated move to heavier weight oils.

Not every vehicle, or every oil change in a single vehicle, shows these levels of degradation with Mobil 1 0W-40; but it happens often enough to be very disconcerting. As a point in fact, one of the very worst M1 0W-40 oil performances we have seen to date was in a new Cadillac that was used as a luxury soccer-mom vehicle; kind of the last place you would expect an oil to crap out quickly. But it did.