You know what.. I have an 06 4.0L and last winter it was getting progressivly harder and harder to start. It felt to me just like dirty intake valves and/or dirty injectors. So, I switched to shell 93 "V power" for about 3-4 tank fulls. I run shell 93 nearly all the time now (have an x-cal w/93 tunes) & it's been fine ever since.
btw, at the time I had about 7-8k miles on the truck.