I use good 'ol Shell Rotela 15-40. I work at Jiffy Lube right now so I get 50% off!!! No more $83.53 oil changes. And I get to top off the leaker all the time which is nice if you have a 7.3, b/c they do tend to leak.
We do have Penn Synth Blend 15-40 that I was thinking about trying on my next change since I'm due. We also have Rotela Full Synth 5-40 that a lot of diesel customers swear by.
Really and truly though, as long as your consistent with changing the oil AND oil filter...your good no matter what you use.