So i am thowing out the questions of at what millage for first oil change, then the interval going forward, what type of oil and viscosity is recommended.
As others have pointed out every one of your questions is answered in the owners manual, it's up to you whether to follow it or not. There is no recommendation in the OM to change the factory fill early but if it makes you feel better to change it early then by all means do it. You'll see early change mileage numbers recommended all over the place, 500, 1000, 1500, just pick one and move on because it really doesn't matter.
As for interval FCA recommends following the OLM and not exceeding 10K or one year. You can follow the OLM or pick a random mileage number like 3K, 5K, 7.5K or anything you want under 10K and change it then, whatever helps you sleep at night.
FCA doesn't recommend or require synthetic be used in the 5.7L, they only recommend 5W-20 that meets Chrysler Material Standard MS-6395. Virtually all 5W-20 oils from the major manufacturers meet MS-6395
except Mobil1. Undoubtedly Mobil1 is good enough to pass the MS-6395 requirements but I've read speculation that SOPUS won't allow Mobil1 to go through the testing process in order to eliminate the competition it would give their Pennzoil Platinum and Ultra Platinum line. Makes good business sense since Pennzoil is the factory fill and service fill at most FCA dealerships.
With the tougher requirements to meet API SP conventional or "dino" 5W-20 has pretty much become a thing of the past. Apparently straight Group II based conventional oils cannot pass the testing so the companies have to add some synthetic base stock in order to pass so they're labelled as synthetic blend or semi synthetic. The percentage of synthetic is irrelevant, all that matters is that it meets API SP, and MS-6395 in the case of the 5.7L. Some older SN and SN+ conventional oils are probably still available at some retailers but most have been replaced by the newer SP/GF-6A.
