Thoughts: Your 10W40 was thickening? Your 10W40 had better basestocks?
There are a number of different VIIP's which radically complicates the entire discussion...as does the avalibility of different base oils. That is, a Group II 15W40 such as Longlife which also uses a fairly advanced VII product will probably show better viscosity retention above 100c than a less sophisticated oil. As I recall, you're comparing to a high mileage blend, so the base stocks (and non-VII adds are radically different).
I can't speak to the Valvoline 15W40...it definitely isn't one of the more standard 15W40's and I have no clue what specs it meets. The more well known 15W40's definitely handle heat better than their dino 10W40 counterparts.
Finally, where is the oil pressure sending unit located? At the end of a gallery? Before or after the oil filter?