What about startup wear? That 10W-40 takes a full 5 minutes to reach all the bearings on startup, but 5W-30 only takes 2/100ths of a second.
Of course, there's always that issue of immediate, catastrophic wear.
Aside from that, while CATERHAM has eloquently stated his case for 10w-30 being an obsolete synthetic grade, I'd argue that 10w-40 is about as obsolete a grade as one could possibly imagine, particularly in conventional. It was terrible oil years ago, and while oil certainly has improved, specifications have changed.
Any vehicles that currently specify a 40 weight oil are either diesels which spec HDEO (10w-40s tend to meet older diesel specs, not modern ones) or European cars, which have their own specifications, which the run of the mill 10w-40 certainly cannot meet. I can't think of any mainstream vehicle currently being sold in North America that would call for 10w-40 as the preferred grade in the manual.
For something well out of warranty that is speced for a 40 weight oil, 10w-40 would be at the bottom of my list of choices, unless it was dirt cheap. Up here at least, 5w-20, 5w-30, and 10w-30 are the grades that go on sale. So, my old truck and the Audi got 15w-40 at a much more attractive price than what 10w-40 goes for.
I know it takes 5 minutes for 10w-40 to reach all the bearings on start up, and probably double that for 15w-40, and maybe triple that north of the border.
The 15w-40 was cheaper though.