2 weeks ago I finally changed my oil and put in Case IH 15W-40 oil.
I've been running Rotella 15W-40 for almost 18 months now, and the truck runs better, idles smoother, and seems to have more power than ever.
This is the stuff, though not where I got it: http://www.hoober.com/product_details.asp?pn=139034A1
You can get it at any Case IH Ag, or Case Construction dealer. It's the same part number for all Case dealers.
A guy that works in another building runs it in his CR Dodge. He claims it stays clean (amber colored) for longer than the Rotella did. I decided to try it, and so far I like it. This is the only change since last fall, and I dropped .100-.200 seconds off my time in the 1/4 mile.