Anyone that has spent much time around the modern Hemi world knows it has inherent issues if not properly maintained. That is the key. Most people just change the oil every 10k miles or more with whatever oil the local shop puts in it, and drive the crap out of them. I have personally seen two 5.7L Hemi's blow up with only about 100k miles on them. It isn't a Toyota V8 or V6 engine so you cannot neglect them, or at least for a long time. I am a long time Toyota guy with trucks, land cruisers and 4Runners, and some went over 300k (why I brought it up). Those engines are bullet proof but gutless and exceedingly boring. The 6.4L with the 4:10 diffs is definitely not gutless and boring, haha. The truck flat gets it done and issues forth beautiful V8 sounds.
I use Pennzoil Ultra Platinum full synthetic that carries the specification for the 6.4 and change it about every 7,500 miles with the end goal of getting 250k out of this truck. The ZF HD transmission has been really great so far and with the massive axles and diffs, the truck is hell for stout.
All of that said, because I was shooting for maximum longevity like you, I special ordered my PW with a minimalist mind set starting with a base model Tradesman, added the 8.4, marker lights, trailer brake control, all vinyl seats and flooring, and the PW package. That is all, no stupid parking warnings blaring and such, and it keeps the truck about as simple as it can be. Some will argue that this isn't needed and the fancy tech on the full PW does not hinder its reliability, but ever had to foot the bill on an older vehicle with gobs of tech when the gremlins come out (and they will)?
An example are Wranglers. I have had a Wranglers for many years and still do. I always buy them new, maintain them properly with full synthetics, and special order them as the most base model you can get (manual windows and all), and they are incredibly reliable. I watch folks in the Jeep club fighting electrical issue all the time I never have, due to the extra tech and gadgets. I smile inwardly as my Wrangler plods along, year after year, problem free.
Flack jacket is properly fitted and ready, lol.