Its a NV4500. It uses syncromesh from the factory but I have used everything from syncromesh to ATF in them. I personally like a 15-40 motor oil in it. In colder climates, you need a lighter weight fluid.
No. The rings inside the unit are either a paper lined ring or a carbon fiber lined ring depending on whether its been rebuilt and who's rings were installed. Other than that, the gears and bearings are nothing different than any other type of manual transmission. Atf would work fine in it but might run a little hotter and be more prone to leaks since the fluid is thinner. It would also allow the gears to chatter a little more in a diesel application. The thicker fluids cushion the gears better and are not as prone to leaks but also don't lubricate under the gears as well as a thinner fluid does. Basically finding that happy medium is what you want to accomplish. Synthetics tends to help with heat build up in heavy duty towing applications as well.
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