I run Rotella 15W40 in my 4.0 all the time. It definitely made it run smoother and quieted down some lifter tap I had. It's pretty warm in Austin so I don't worry about the cold weight being a little thicker. It also stays cleaner longer. I had been using Castrol 10w40 and it would get visibly darker after a month or so. I've been running the same Rotella in there for about six months now and it's still pretty clear.