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I do 6 quarts in mine but you can barely get 5 in through the fill port on the side. You either have to park sideways in a ditch (preferably one you can drive back out of) or pull the shifter from the top of the trans (not hard to do at all). I also use the Amsoil Syncromesh stuff.
This has been recommended to be by several people, including a good friend who is the local Cummins/Dodge tech. Reason is, when pulling a steep hill on mountain highways and such, the trans oil will flow to the back of the trans and leave the front bearings dry. With 6-quarts in, this won't happen and it does NOT create any over-fill problems.
If you don't pull the PTO covers, you can only get a little more than 4 quarts in on the refill...a lot hangs up in the "sump" in the bottom. I usually pull the cover, then soak up the excess with towels. Then it takes just shy of 5 quarts to refill.
Synchromesh is the fluid, but they are not all the same, and act differently in each NV5600. One may work better than another in your particular trans. Mine likes Redline the best...didn't shift for beans with RP SynchroMax, and Amsoil MTF shift quality was short lived. The only way you will know is to pick one, and use it a while...then try another if the first one didn't work well.
Overfilling has been debated...all I can tell you is that overfilling by a quart (before coolers) caused the oil to run hotter in my NV5600.