Hot take: stop putting 15W-40 in everything, a 6.0 Powerstroke is not a 12 valve
I keep seeing guys dump 15W-40 into 6.0 and 6.4 Powerstrokes at the yard in Amarillo because that is what the old 7.3 liked, and then they wonder why the oil rail and injectors act up at 40 degrees. Ford calls for 5W-40 or 10W-30 in those motors for a reason, the HEUI system shears oil way harder and cold thick oil is the quickest way to kill injectors. If you are working on a 6.0, check the cap and the manual, not what the last guy ran. What weight are you all actually running in your 6.0 trucks up north?
Isn't the bigger issue that Ford actually called for 15W-40 in the early 6.0 trucks, then switched to 10W-30 and 5W-40 later on? I have a 2004 and the cap right on the motor says 15W-40, so guys running it are not crazy, they are just going off what their truck came with. The HEUI thing is real though, cold thick oil does beat up the injectors and the spool valves, and the later Ford spec moved to thinner stuff for exactly that reason. Up north here in the cold months most of us run 5W-40 synthetic and it makes a big difference on cold starts. So I get what you are saying, but saying Ford calls for 5W-40 across the board is not true for every year. Check the year and the cap, not just one rule for all of them.