At the time I originally created this writeup, I used the capacity that had been listed elsewhere because I couldn't recall exactly how much I had used. For the record, it takes just a bit less than 2 quarts of ATF when you simply do a "dump & fill" fluid service.
The type of ATF to use is 100% irrelevant, I recommend to use the cheapest available. Different types of ATF exist because of the different requirements needed in the friction modifier aspects of the different fluids, specifically with respect to the friction materials of the bands & clutch plates. Your transfer case does not contain any such material, therefore the friction additives in the different types of fluid are without consequence. Simple lubrication is the only goal.
Just a note, if you don't have metrics that big with you when you might need to do this ( for example after having to have a logger pull you out of an impromptu mud hole with his skidder). a 12pt 1 3/16 socket will fit the drain and fill plugs nicely.
Here is how I refilled mine for anyone else that had troubles at first. Short wide funnel and a short piece of vinyl tubing. I had just enough between the extension housing and the tub to empty a whole bottle with ease. warning the tubing I used was fit snug in the fill port and I managed to over fill the case by a far amount and wasted a good deal into my oil pan.
It doesn't have to be ATF4, you can use ANY brand, type, flavor, etc. ATF that you want. The only difference between ATF specifications is the additives package, concerning how the fluid reacts to the friction material inside the transmission. Your NP231 does not have any such friction material, the fluid provides only basic lubrication.
Hey Max! Thanks for all of the advice! Did this today. Seriously took me no more than 20 min! One thing I didn't do was scrape to the right to check the magnet for shavings. Should I go back in there and clean that magnet? Also the oil that came out before, definitely was not ATF. It looked like motor oil. Is it ok to mix ATF with motor oil in the TC?
Also, mine had Allen bolts instead of 30mm bolts. Just saying..
so what do i do if the owner before me put gear oil in the t-case? should i just drane and refill with aft+ or what. Plz help im rebuilding in now and want to know if i should clean out as much as i can or will some gear oil not matter?