Use Power Steering Fluid or ATF in power steering pump?

[ShinoBino]

I've owned this P5 for about 4 yrs now and in about the 1st year I decided to change the power steering fluid as it had a dark brown color. I flushed the reservoir with about a quart of autozone Power Steering fluid, till the fluid was clear enough for me. With the years going I got a couple power steering problems but I believe none came from using power steering fluid instead of ATF. I had a power steering leak because of a rip in the high pressure line, I replaced the line. Then Replaced the belt because it tore apart in the engine bay randomly on the street. Now my steering feels hard and the fluid is dark again. I thought of flushing it out again to fix the hard steering and longevity, so I was looking at some videos on how to do a complete flush and ATF was used instead. After looking through the owners manual it specifies the P5 needs ATF M-III or equivalent (e.g. Dexron® III). I have been running power steering fluid with no real problems for years now so should i switch to ATF like it specifies or PSF because no problems and its cheaper? (at least in my area its cheaper) And if i need to switch is ok to have the fluids mix. Im thinking as long as i flush it till i see red ATF, all the old PSF should be cleared away right?