This is from a NV4500 & NV3550 manual I downloaded...be warned...and this is from Advanced Adapters - not dealerships. I had to to buy some for my 4500 :'(
TRANSMISSION LUBRICATION:
The NV4500 transmission uses approximately one gallon of synthetic oil. This oil is available at your local Dodge authorized
dealer under Part No. 4874459 (quart), or your local GM authorized dealer under Part No. 12346190 (quart). Unlike regular
petroleum oil that breaks down when it gets too hot, this synthetic oil is made up of different properties that do not break down.
The transmissions are also equipped with synchronizer rings that are made of a fibrous material. The wrong fluid will cause the
unit to develop “sticky” shifting. If the wrong fluid is used, more than likely you will have to have your transmission rebuilt.
DO NOT SUBSTITUTE any other lubricant as internal damage will occur.
NOTE: Castrol Syntech is NOT the same as Castrol Syntorq.