use type F ford atf, on some of the older fords using regular power steering fluid is actually bad for your car. Of if you are really worried about your ps system, add some lucas into it. That stuff is magic in a bottle, I've seen it do some amazing things first hand!
Transmission fluids are different due to "friction modifiers" that are part of the blend.
An Automatic transmission has "wet clutches" that run in the oil similar to a most modern motorcycle clutches (not exotic race stuff, or "dry clutch", etc). The transmission fluid needs to be matched to the clutch type in the transmission, thus the different types "Type F", Dextron, etc. are blended with friction modifiers to match the needs of the clutch....
In a power steering system, there are no clutches or friction plates, so I can't see how any of the trans fluid would hurt anything. Probably any 10wt non detergent oil would be fine. I think you could even get away with 30wt non detergent oil in a pench - at least in Florida during the summer...
Oh, if you ever wondered why transmission fluid is red (or Royal Purple is Purple), it is only a dye added to the oil to identify the type - the color has nothing to do with anything important.
There may be some other additives for seals and such, but I've always used Dex 3/Mercon in my vehicles. It always seems to be on hand, compared to p/s fluid.
As for the statement from Hal, if you were to lift the hoods of a Freaightliner, Pete, KW, or even the Volvo, one truck specifies 15w-40, one may spec P/S fluid. They use the engine oil figuring a driver would have that on hand (always carry a spare gallon to top off the engine) compared to having another bottle of P/S fluid too. They have to haul everything everywhere, so less to carry = more living space.
1978 CJ5 5.0HO/T177/D300, '86 D30/D44 WT axles, 'glass body, 31x10.5 BFG A/T, TDK galv'd frame - DD and weekend toy