If I'm not mistaken, the manual calls for 10W-30 for regular driving. Seemed odd for a diesel but they're smarter than I am for that.
I like the 5W-40 synthetic for my peace of mind. Yes, its more expensive but:
1. I feel more confident going until the computer tells me to change the oil.
2. I tow heavy with this truck 90% of the time its on the road.
3. I drive less than 7500 miles per year on this truck so I like having synthetic sitting in there for an entire year versus conventional oil.
Just my .02.