Fred,
I have a 99 F350 powerstroke, and prefer to run 10W-30 Rotella in the colder months. My truck does not like the weight of the 15W-40 whens its cold out, I have had the cold start problem, even after check glow plugs, relays, and plugging it in. Once I changed to 10-30, no more cold start problems, I don't even have to plug it in. The hardest thing is finding someone around my neck of the woods that can get it, no body seems to have it or get it.
John