You could try some Marvel Mystery Oil in your oil to see if you can free anything that may be gunky. But if you are going to change all your seals, you may as well just clean out any caked on crap from under your valve covers while you're in there, and check for potentially loose items in there as well. Maybe its an exhaust leak and it sounds similar enough to you, but very very not likely because you'd DEFINITELY notice it when accelerating, even if it was faint.. Milky oil sounds like a leak somewhere for sure, especially if you are losing coolant, and its not from somewhere obvious.
Best bet as always (if money and vehicle downtime isnt a major deal) would be to dig into it just a bit so you can inspect things yourself, and give it a good look over. These engines wont let you down, if you don't let it down on maintenance and regular inspection.
I worked on my 5.9 that I got ~140k in rusted exterior condition out of Minnesota, derusted the outside related cleaning and painted it, new seals where needed, cleaned any bad carbon buildup (was very clean overall) and then strapped her back together. Never had a stronger and more reliable motor, already put like 1500 miles on it in 1-2 months just around town.
If you were able to post a link to a video of your truck running/idling we could hear the noise better and be able to help out more too probably!
Its also possible if it sat in a field that long, there could be grass or other crap that grew up and into the exhaust, that would certainly make it sound poor haha, Ive seen my share of exhausts that have dried grass all in them, among other tragic muscle car stories involving sitting in fields
Also in relation to damage from overheating, my father has been a mechanic all his life, and he said a cop told him rather recently, they used to use an old Dakota 5.9 as their ramming vehicle, and they ran it for QUITE a often in overheated conditions, and if I remember right I think he said the cop told him the engine didn't actually go until 30+ minutes of overheated operation one time, couldn't tell you how much of a stretch his story is, but I've definitely heard my fair share of these magnums reliability.
Long winded, but hope that is somehow useful.