With all respect sir, what hemimann is saying is from his understanding of the life span of 0w40, in fact proven with uoa's at ram forum, 0w40 PUP the intended oil for the 6.4, spends like less then 10-20% of it's lifespan in 40 weight on a 10k mile interval. So literally 80% of the time that 0w40 is actually a 5w30. Many were posted here lately, mind you this is a 4k mile interval, note the cSt viscosity in 3 uoa's, it is in 30 weight below 12.25. If FCA wanted the 6.4 in 40 weight, they would have recomended 15w40 like everyone else. Manu's are using 0w40 for a different purpose lately. Many other uoa's show 0w40 PUP in the 10's with more miles, you go long enough that 0w40 will be a 20 weight. Because the 0w40 PUP isnt dexos or other stay in grade specs, it is full of vii's that shear and are useless molecules not helping lubrication. Why someone would use a 30 weight instead of 0w40 in a 6.4 is to have less vii's. It doesnt matter in the middle of the interval whether you chose 0w40 or 0w30 or 5w30 or 10w30, in the middle of the interval the weight will be 30 weight, FACT. Another fact, all of those 30 weights will be lubricating better because they will have less vii's, so less useless molecules, fact 2 and why hemimann says what he says.