The Amsoil OE is a good oil. I would have no problem using it. I use another brand, but I am not brand loyal as some are and worship at their oil alter regarding the brand they use.
Now the dexos1 thing. If you check your owners manual, it says "dexos1 or equivalent". At least my 2013 1500 manual said that. Now, there is a TSB out from GM themselves, that categorically states that if dexos1 approved oil is not available, then a quality synthetic that meets the GF-5 spec is adequate. So you have two examples, straight from GM themselves, that while they want you to use a dexos licensed oil, they do not require it.
Oh.... the TSB from GM is 13-00-90-001. It even goes so far as to say this same thing on the newer Ecotec motors that use the 20w oils.
Look, they put this stuff out there to prevent users from buying some two bit no-name oil made from dog snot and putting it in their motor. I know some would use the Magnussen Moss Warranty act as an argument that you do not have to use a dexos licensed oil, but we don't even have to go that far. GM themselves acknowledges that a quality synthetic that meets the ILSAC GF-5 spec is adequate. There is nothing going on inside of GM's motors that is all that much different than any other brand. I would contend that most any synthetic or synthetic blend that meets the API SN "Resource Conserving" and the ILSAC GF-5 specs, which are the latest industry oil standards, will work fine in the pickup.
I never used a dexos "approved"oil in my 2013 1500 5.3L or my 2015 2500HD 6.0L. I used the Schaeffer 9000 5w30 full synthetic in both of these vehicles. Never had any concern whatsoever. It met API SN, ILSAC GF-5 and dexos1 specs.
Truth be told, the dexos1 only differs a little from the other specs in two areas.... it requires a maximum of 12 NOACK, or burn off rate, and it has a little higher anti oxidation requirement. Interesting, that GM pushes the ethanol thing is a major way, the GF-5 spec regarding ethanol far exceeds the dexos1 spec when it comes to ethanol. Look at any "spider" diagram that overlaps the various specs and shows their strengths and weaknesses.