The manual for my truck says 5w40 during the winter is fine. It has a temperature chart if you're interested? I can't recall what it says off the top of my head but, 5w40 is recommended for cold weather.
2008 F350 CCLB Work in progress
'96 F250 ExCab 4x4
4" Exhaust Tony Wildman Chip Electric Fuel 17* HPOP Swamp's IDM Tymar 4" Lift Smoked Headlights 5000k HID's Front End 315/75R16 BFG All Terrains Optima Red Tops
It's still a 40 weight oil at higher temperatures, it just flows like a 5 weight when cold. I guarantee you there are more people having problems with 15w40 Rotella than any of the good 5w40 oils. Personally I use the Valvoline Premium Blue Extreme 5w40 and my truck has never been happier.