Hello E Money,
I have been using Amsoil for twenty plus years. I have had excellent results with all applications. Like one of the other members here, I use it in everything I own. I have a ten year old lawn mower that is Amsoil equipped and it starts every year on the first pull.
Keep one thing in mind. Amsoil was the original synthetic oil. All the other companies that sell it have copied the original. Amsoil is not in any other type of business besides producing quality synthetic lubricants. As to MPG increases, you are not likely to get a huge increase in mpg from the change. Smaller displacement engines seem to respond better, but then anything you do to a small displacement engine is more noticable.
Amsoil will make your Duramax last longer and run better. It's that simple. Amsoil is not cheap, but if you were looking to cut corners you would never have asked the question about synthetics. Many of the major brands have TV and magazine ads for their "synthetic" products but if you watch many of the off road racing shows you can count the Amsoil decals. It speaks volumes about what the racers trust that are not dipping into the pockets of an advertiser.
Amsoil is without a doubt the way to go. My truck is new, and has less than 1,000 miles, so I have not made the switch but you can bet the first change will be to 5W-40 Amsoil Diesel.
Bamamax
2009 C-3500