I've started the 04. 5 down to -20*F unaided... as someone else said, it wasn't happy but it fired and ran nonetheless. Pretty sure this was a updated reflash at some point (it didn't do this when I first bought it) but mine starts in high idle at any temps below 20*F... it did it this AM and it was only 13*F.
One thing that I've noticed, and I saw lots of others saying the same thing, is that the truck starts a lot easier with 5w40 (synthetic), and I get oil pressure a lot quicker. You can tell the starter doesn't strain as badly with 5w40 in the sump.