5W-40 typically gives better startups, especially when cold. Just flows better cold. Now that Dino oil. Synthetics are even better flow cold. Only downside to running a 5W40 compared to a 15W40 is there are more thickening additives in teh oil to make that 5 weight act like a 40 weight.
Yup. I run Amsoil 5W30 in my cummins, but I reccomend the Dmax guys stay with the 40wight oils. After seeing the oil pressure and overheating problems the select few LLY's had, I would hate to risk lowering the oil pressure and more.