I echo everything Mark said, BUT
being the cheap SOB I am I still went ahead and ran ATF in the power steering system. My hydro is assist not full hydro. I ran ATF in the XJ as long as I can remember, one steering box, two pumps. I think both were due to normal wear and tear, big tires etc. NOT due to running ATF instead of PS fluid.
SO if you are a cheap or lazy SOB - run the ATF. I have also seen farm hydraulic fluid used, to no ill effect, but I wonder about heat transfer with that stuff.