The issue with biodiesel is less its viscosity (particularly in the hot months), and has to do more with its solvent properties and compatibility with rubber hoses, seals, and the like. Any diesel engine will burn biodiesel fine, even at B100 levels. The question is how long all the ancillary parts will last.
I don't have any big-truck experience, but I ran homemade B100 in an IDI Mercedes (W210-era E300) at about 2000psi injection pressures -- over 75k miles with no problems other than the occasional replacement of a small fuel return line weakened by the biodiesel. I later ran my same homemade B100 in a TDI Volkswagen (V10 TDI Touareg) with 30,000+ psi injection pressures -- over 50k without any problems at all. My current Cummins Dodge (6.7) sees lower blends, as I just don't have the same amount of time to make fuel as I had in the past.
You do have to watch cloud points and gelling temperatures depending on your climate and what feedstock the biodiesel is made from when you're running it straight or at very high blend levels. I've only had "frozen fuel" once, in an unexpected snap freeze when I hadn't made time to go splash a couple gallons of D2 in the tank. Lesson learned!
Cheers, John