Everyone in this thread already agreed that you should move away from Rotella T6 5w-40. What else do we need to say?
Whether the Duron 15w-40 will work better or not is anyone's guess. You won't know until you try it. We don't have any first hand knowledge of this oil here, so you just have to be the first one to try and let us know how it goes. I'm guessing it'll be fine.
If you'd rather not experiment and use something that we know works well, sunruh listed those options earlier in this thread for you.