OK, I will say the opposite, so don't hate everyone.
I run the 5W40, specifically Amsoil, in my two-gen RAMS only because it's Amsoil.
Their cold flow test and general tests are impressive, but for some reason, 5W40 is the only weight oil they list for my 2016 and 2018.
Like you, I remember wondering if it was OK to run that oil year-round.
I called them, and they said yes, it is absolutely.
I also run it with their bypass system 60k mile filter.
I am not trying to sell you anything.
I prefer their oils based on the results of independent tests I have seen over the years and on my vehicles.
Plenty of other great oils are out there, and if you want to stick with 15W40, stick with it.
I can't say I have seen mileage increase or anything, but my Blackstone lab tests looked awesome before testing this oil.
I will say that if you have a 5th Gen, I won't screw around with any other weight of oil than what they call for.
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