I was still in the dealership when they rolled out the +4 to update +3. No issue with backwards compatibility, FCA has published countless TSB's speaking to the compatibility and application of +4 for AT, power steering, t-case, etc., covering a LOT of years. Been too long since I looked, but +4 is either a semi or full synthetic as well, definitely handles some heat with solid reliability. For sourcing it, go hit your local walmart...they have Mopar +4 on the shelf, cheaper than most stealerships.
For me, I'm on the Mobil1 full synthetic train for most fluids, had really solid luck and they aren't cost prohibitive, unlike Redline, Amsoil, etc.
As for the parts guys...yeah, that gig with most of the newer generation of parts guys sucks. I was slinging parts, handling warranty stuff, delivering, basically a one stop grunt/slinger for close to 2 years. Our guys were legit, old-timers, had one guy that could rattle off part numbers for a 69 D200 taillight from memory. Got a new guy in, totally clueless. Had to deal with the FNG's with a dealer I sometimes go to now, sucks most of the time since the older guys aren't on the phone anymore.
Damn I sound old and I'm only 40...lol