I've got to agree with rgl about using a good synthetic or really good semi synthetic ATF(Schaeffers #204S
) for the transfer case. With the chain driven transfer case a gear oil is most likely to be too thick and cause problems. Change it every 30,000-50,000 miles, depending upon 4X4 usage and you should be fine. I looked at the manual for my 2002 F150 4X4 and they don't even mention changing the transfer case fluid ever
! Not a good idea if you want to keep the truck for long time. It's a cheap and easy maintenace to do. A lot cheaper than replacing the transfer case.
Whimsey
Whimsey