We're tracking MPGs with different oil additives, and without.
I can say without hesitation, MotorKote is coming out on top, followed by Aimsoil & HotShot (equal), and last is straight oil. Typically we're seeing 1.5mpg improvement over straight oil, everything in the truck is stock (except tires are just slightly larger).
As you suggested, our sweet spot for best MPG is +-65mph, maybe closer to 70mph.
Regens just KILL mpgs, like Diesel #1 drops us at least 1 mpg, so we always look for Diesel #2 (the down side is more regens, but the fuel economy makes up for it).
We recently added MK to power steering and transmission, and then hit a F-king deer, a nice big buck who hit perfectly flat against the front, missing both headlights, but crushing the intercoolers, trans-cooler, bent the radiator just barely, and ran up a $11,350 repair bill. As soon as it gets out of the shop, the truck gets a grill guard, whether I make it, or buy it, the damage to the front end - which looked minor - was extensive and expensive! And.. the 8 point buck, shook it off, and ran down the highway.. didn't even lose a drop of blood, patch of hair, etc., all damage was wifey's truck.