Bill:
Great post.
I am going to address #7 specifically:
When I ran the Mobil 1 Turbo Diesel Truck in MY Expedition, I noticed a continuation of the carbon deposit trends in my oil filter. This would indicate to me, that it is performing the same "level" of cleaning that my maiden run of 5w20 did. But the engine is quieter on the 5w40, and the big thing for me was the elimination of consumption.
From my results with the 0w40 in the Lincoln (and really, this oil is barely a 40-weight anyways) I feel confident that it would be providing perhaps some better protection than the 5w20, while providing the same, if not a better level of cleaning.
I have no idea where the carbon is coming from or why it is there and that is a genuine concern. This engine should not have ANY buildup of ANYTHING in it given its history.
Maybe there is something to the argument that thinner oil gets into the ring lands. Though I can't see the rather small difference between the 5w20 and 5w30 showing up as such a LARGE difference in what I've now observed with this filter and oil.
There are a lot of questions and I would be lying if I said it didn't bother me to not know the answers to them.