I have been working on NP231 transfer cases since my first YJ I bought in 1995. I have always added Mobil 1 full synthetic ATF, and never had a problem. The manuals have always called for Dexron III ATF, which the Mobil 1 exceeds the specs.
I was changing the fluid in the Liberty's NP231, and I checked the manual for the fluid type while I was waiting for it to drain. The Liberty calls for ATF+4 (full synthetic). I know Chrysler changed the spec of the transmission fluid from DexIII to ATF+4 in 2002 and I am guessing they changed the spec for the transfer case fluid to ATF+4 so the dealer only has to carry one type of fluid....or it could be a scam to charge more for the ATF+4 fluid which is about $6 a quart.
It's no wonder why Chrysler is headed for bankruptcy.
