Welcome! My old 6.4 also sounded like a thrashing machine on Amsoil SS, so you are not alone. I had success with Redline 5W40, but wish I had also tried 5W30 for comparison. I prefer using an oil that keeps it happy, rather than play additive roulette, personally.Hello guys long time lurker and just joined. I’ve got a 2022 Hemi 6.4 and one of the things I noticed right away when I bought this truck over a year ago was the lifter noise on start up when the truck has sat a while that only lasts a couple seconds, in my life owning new trucks I’ve never heard such a thing and immediately thought I could remedy it by using something other than PUP. I order Amsoil 0w40 and a Wix XP filter and it only made it worse. With Amsoil it did it almost every start where with PUP it was here and there but not every start. Immediately got some more PUP and an OEM filter and dumped the Amsoil. After doing some research online about this issue I saw that a lot of guys were also running Scheffers Moly EP with good results so I thought I’d give it a try. Using PUP with 1 pint of Schaeffers Moly EP really helped quiet down the cold start lifter chatter. I don’t see a lot of guys on here talk about the Schaefers Moly treatment and was curious if anyone else has been running it.
On Edit:
To clarify, I don't own the 6.4 anymore, am back in a Cummins, so just passing along previous experience.