I flushed my PS not very long ago, when I heard the "PS whine" that was the idler pulley. I bought a gallon of the store brand PS fluid at Advance or Autozone. It's super easy to remove the one bottom hose from the cooler, and stick it down into a large pan(wrench lightly on the end holding it down well).
Then most of the fluid will drain, but you can quickly when ready, start the engine and pour the cheap clean fluid in, no harm as long as fluid is in the reservoir. Shut if off, replace the hose, and refill.
I've never used ATF for the PS, not since the early 80's with my old Gran Torino. I always use PS fluid for the PS system, I've seen a bunch of Fox Mustangs with terrible rack problems, mostly leaking, but whining or grinding etc. Those cannot be all from random bad PS pumps or racks. The cheap ATF people use is more likely to harm, than any PS fluid.
I've only used the Amsoil ATF that they now call OEM/universal. That's all they had before last year, now the Signature is new, and another choice I don't recall. It's all very good. I like their PS fluid, that's what I like to use, but it is expensive.
BTW, wee do have some local sources for Amsoil, dealers. I haven't contacted one yet, but I noticed them the last time I was hunting Amsoil online.
