The argument for 5w20 redline to 5w30 redline would take a couple pages anyhow, just from the RAM forum members results against hemi tick. Of note, the first time I killed my own tick which means the first success we had recorded from an oil strategy was on 5w20 redline, but then as we went through the field of Rams it was hit and miss, but 5w30 was more hit then miss. The oil pump is an equalizer, it pushes oil similar to a water wheel, as long as the water basket is full, it pushes the same amount of oil no matter what the weight, especially at operating temps. The real world RAM forum to a man proves 5w30 gains you around 2psi over 5w20 redline, it was the forum that convinced me to abandon 5w30 redline for more then a couple reasons that even included off brand vehicles and my own testing. Again we are talking pages of typing but trust it. The one knock on 5w30 redline has been in cold places getting cold piston slap not related to hemi tick. In those trucks 0w30 redline was a much better choice.
Ask hemimann with his 6/4 I do believe he was on 0w30 redline in a snow state. Since then he is on 0w30 HPL SC that also has a forum discount. Long answer short, eventhough redline has extra protection from other oils, no I wouldn't run 5w20 in a 6.4, but there are options if you developed cold piston slap. 30 weight in a 6.4 is not an issue due to the OEM oil for 6.4 which is pup 0w40, most of every uoa we have it at 10 visc, so as long as you match that (end on interval visc) you are safe. The right 30 weight does better then matching that, as in 5w30 redline usually is still in the 12's but yes sometimes the 11's depending on the shear of any particular engine, still a full point about 0w40 PUP at the end of a run. 0w40 is a very unstable viscosity, so many guys are running stout 30's instead. Of course the other hand would be instead of going down, can you go up to 5w40 or 15w40 in the 6.4. The issue here is start up wear, you will get more start up wear over what they designed to run especially in cold states. So either follow the manu or use something proven to stay in grade similar to manu oils, but don't go picking a shelf 30 weight, something like pup 5w30 which I do like myself in my Toyota, would be a very bad choice in the 6.4, it would be a 20 weight in short order.