Since you are already leaning towards Amsoil, I'd like to share a few opinions on the topic:
1) Wear rate alone is not a reason to switch to synthetic for our application. Delo users get very low wear rates, and the Rotella users aren't far behind, and the engine outlasts the truck with any quality diesel oil. I'm a big fan of synthetic anyway for a host of other reasons (cold weather starting, longer change interval, less worry about turbo bearing coking, etc, etc, etc); I just don't want you to jump on the bandwagon with me and then feel like we tricked you later!
2) Get a bypass filter if you're going to go longer than 5k on your changes. Many use the dual bypass from Amsoil (including me), but I'm going to switch to the oilguard or Amsoil single bypass. While many go 10k, 15k, or longer without bypasses, my theory is that bypass filters have more impact on wear than synthetic oils, so why do one without the other? And why risk increased wear with long intervals and no bypass? I could go on and on and on, but it's already there in old threads so I'll move on...
3) Amsoil 5W-30 vs 15W-40: A number of us who started with the 5W-30 have switched to the 15W-40; many others just started with 15W-40 in the first place. 15W-40 users report less oil in the blow-by bottle and less "misting" from the blow by bottle (you'll see this in the headlights at night when sitting, sometimes). The 15W-40 is the weight recommended by Cummins. Several reported lower wear rates with 15W-40 (although at least one or two reported lower rates with 5W-30, so you may have to do your own testing). Finally, 15W-40 is much more economical, essentially ending any temptation to play with long drain intervals unless you have the bypass (a 10k run, with a filter at 5k, will treat your engine right, and you could afford 5k changes if you're a low mileage driver). With all that said, your signature mentioned snowmobiles, so, your needs might be different than mine and 5W-30 might be the right choice for you.
4) Yes there have been heated debates in the past. One good point I learned was that if you have significant modifications to your engine, fueling levels might dictate short changes no matter what oil you use. We tend to have these discussions without asking for application details that can change what is the "best" answer. Bottom line, if you extend your intervals, do oil analysis and learn what's right for your engine, your application, your number of cold starts / short trips, and to be sure you don't have an antifreeze leak sneak up and kill your oil.
5) Oil filters: The one you want is the LF3894. Buy from your local truck place or order from Geno's on line. Keep your receipts. Print a copy of this TSB:
http://www.dodgeram.org/tech/tsb/2001/09-004-01.htm
With both of those in hand, Dodge will not be able to give you any grief over warranty issues. Footnote: This is another reason I think those under warranty might be better with a single remote or Oilguard bypass filter - Amsoil filters are good, no doubt, but it is a smaller company and their filters do not appear on the "approved" list. With the Oilguard, single remote, or other true "bypass" style filter, you can keep an "approved" filter as your full flow filter, while still enjoying the benefits of bypass filtration.
6) Warranty in general: I would recommend that you don't specifically tell the dealer you are using Amsoil, for two reasons: first, there is a technical/ legal issue because Amsoil has not paid for API certification, and second, because believe it or not, a small percentage of Amsoil users have given the oil a bad name in some circles by doing stupid stuff like switching to Amsoil and then running 20k or 30k with no filter changes, no oil analysis, nothing. Even Amsoil can not magically shield the mighty Cummins from the full power of stupidity!
Attention: TDR Forum Junkies 
All you need is a Fram filter w/generic oil and you'll be fine! You can go at least 15000 between changes
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