I used Amsoil in my 99 until about a year ago, because in 1998 when I bought my truck it was pretty much the only one available. Now I use Rotella, Valvoline purple, Shell, depending on what I can find the cheapest. (I will have to read the article by Robert and John. ) In my opinion as long as it is synthetic and a good brand (shell, rotella, valvoline) it pretty much is the same except for price. I use synthetic because it is a proven better lubricant then dino oil, and although like West Tennessee said conventional oils will do the job, synthetic does it better. We all spend a lot of money on these trucks; add ons, HP increasers, etc so a few extra bucks for a better oil is well worth the money.
Amsoil needs to lower their prices; they are no longer the only kids on the block.