Originally Posted by N_Jay
So is it ATF (or the proper kind), or Gear Oil (of the proper kind)?
Assuming your car is a slushbox:
Front diff takes gear oil. Automatic section takes ATF. Center diff takes gear oil. Rear diff takes gear oil.
Front, auto, center sections are combined into one unit with three distinct chambers. If you have ATF in your center diff, either someone installed the wrong fluid or you have an internal seal failure.