All of the trucks we have bought have had "10W-40" stenciled on the pre-heater cover. We do not wish to experiment with anything other than that weight due to possibility of damage. We use our old standby Rotella-T 15-40. The new International engines in the State Highway trucks my son works on specify Rotella T 15-40 or Shell Rimula. The manual specifically states that to use any other oil voids the warranty. I would not experiment with going to a different weight oil for the possible oil price difference, but I do not think that is what you are saying here.
And before my statements ignite another long discussion on oil types, I am not saying there are not other good oil brands as well as Shell Rotella 15-40. I just believe in it from over 40 years of using it in all farm, pickup, racing, military, and heavy equipment engines I have owned with excellent success.
Regards Marti