I use good ole Rotella T 15w40 is this good enough? What would the advatage to going to Rotella Synthetic?

 

The synthetic flows much better at sub-freezing temps and gets slighty better fuel mpg. My truck starts quicker and runs much quieter (cold) since switching to Rotella 5-40 "synthetic". I originally planned to run the 5-40 only for the winter but after seeing the difference it made, I may run it all year.