As a long time member at BITOG (since 2004) I see this question asked dozens and dozens of times a year and the answers are consistently all over the place. You see recommendations to dump the factory fill at 500, 1000, 1500, 5,000 miles or leave it in until the OLM gets to 75% or 50% or 20% or even 0% and everything in between. They are for the most part just random, arbitrarily chosen numbers. One poster in this thread said the dealership told him to bring it in when the OLM hit 40%. Why did the dealership decide 40% was the magical number and not 50%, or 45% or 38%? When I picked up my new Laramie the sticker on the windshield said to bring it in at 5,000 miles for an oil change. That goes against what FCA recommends in the owners manual and is just a random number pulled out of their rear end. If two people have the same vehicle and one dumps the ff at 500 miles and the other dumps theirs at 5,000 or when the OLM is taken down to 0% does that mean the engine in the one done at 500 miles will run better or last longer than the other one? Without extensive long term testing no one can answer that. The op would be just as well off having one of his kids or spouse or co-worker just pick a random number between 1 and 10,000, and change it at that mileage, or pick a number between 0 and 100 and change the ff when the OLM reaches that percentage.
The second question regarding when to switch to synthetic also gets recommendations all over the map. So far in this thread we've been told that these trucks come from the factory with some kind of magical break-in oil, we've also been told it's just plain old Pennzoil semi synthetic and we've been told the ff is full synthetic. So, which is it? The idea that an engine needs to be run on conventional for some arbitrary number of miles before switching to synthetic has no basis in fact, and that is easily proven by looking at vehicles that require 0W-20. FCA, GM, Honda, Toyota and other manufacturers have sold untold millions of regular, everyday cars, SUV's and pickups for many years that come from the factory with 0W-20, including the 3.6L Pentastar. Because of the cold pumping requirements of 0W-20 oils they cannot be made using conventional base stock.
None of the major oil companies list a conventional 0W-20. Some do list a 0W-20 synthetic blend but the overwhelming majority of 0W-20 oils on the shelves at Walmart, Advance Auto, O'Reilly's etc that diyers use are full synthetic. The only synthetic blend 0W-20 Valvoline lists is their Maxlife high mileage but that's not what most people are going to choose for a relatively new,low mileage vehicle. Pennzoil has their Gold synthetic blend but it's not easy to find, the shelves are generally full of Platinum and Ultra Platinum so there's no point chasing down the hard to find synthetic blend 0W20. Honda used to offer their brand in a synthetic blend 0W-20 in addition to their full synthetic but I don't know if they still do or not. So, in summary, if you purchase a vehicle that requires 0W-20 it's going to come from the factory with at a minimum synthetic blend, and more than likely a full synthetic, so when it's time to dump the factory fill there is no option to run a conventional oil until the engine is broken in, whatever that is, before switching to full synthetic.
Of course, high performance vehicles like the Corvette and SRTs come with full synthetic 0W-40 and since there is no conventional or synthetic blend oil in that grade those engines live their whole life on full synthetic oil. There is no reason to believe that there is anything special about the 5.7L that requires some arbitrary break in period using conventional or synthetic blend before switching to full synthetic when you consider the millions of engines out there that have had nothing but synthetic their whole life, including the ones built by FCA, that run for hundreds of thousands of miles.