I chose Rotella Syn. 5W-40 for the following reasons... but had used Motorcraft 15W-40 for the first 50,000 miles with zero problems.
1) I live in Kansas City and need better cold weather flow properties of syn.
2) I like the hot and cold properties of syn. after reading about them over the years.
3) I can readily buy it at Walmart as some other quality brands are only available at certain distributors and outlets.
4) Rotella - Syn. is CI4+ rated and this is the newest highest rating for diesel oils (other oils have it to, but read the lables).
5) I picked up 1mpg after switching.
6) I change oil and filter every 5,000 miles regardless of how I have driven and regardless of oil brand or type used.
Good luck on what you choose. A name brand quality oil that is CI4+ rated will do you right.