The company I work for runs a fleet of modulars, mostly 6.8L 2v engines. They're on loaded F350/F450's, thus they see their fair share of abuse.
They've used every oil weight under the sun, yet they run forever and never break down. The first mechanic was an old-timer that swore by 10w40 and his reasoning for the OEM suggesting 5w-20, is that the truck is built in Michigan where it's cold. News to me, as the trucks are actually made in Kentucky. That mechanic is gone now, so the operators usually pick the oil weight now. Lately it's been 5w-20, and the engines just don't care.
Personally, I think if you value the utmost in fuel economy and efficiency, run 5w-20. If you want to go the distance and drive the darn thing to 300k, I think a heavier weight would be beneficial toward longevity. I like HDEO oils myself, and if I owned a truck with this engine, I'd use a 10w-30 HDEO like Chevron Delo XLE. Or Motorcraft. Or Shell Rotella....