Most older power steering systems are basically hydraulic systems, a belt driven pump, some valving, and an actuator in your steering box or rack, power steering fluid is basically hydraulic fluid. a light oil with some antiwear additives and corrosion inhibitors, possibly some anti-foam additives,
ATF has similarities to a hydraulic fluid, usable in many systems and actually specified in some others. when I was a kid I never used power steering fluid, ATF was on hand and worked fine, one less fluid to have around.
I good results in an 80 series land cruiser using mobil DTE 24 in the power steering system that specified Dextron III , DTE 24 is a high quality multipurpose hydraulic and gearbox lubricant for machine shop tools like lathes and mills, it got rid of pump noise when at the lock and gave better steering feel,
on newer vehicles with electric and clutched power steering systems designed to reduce fuel consumption I would take a close look at the specified fluid before I tried something else. some are not nearly as basic as systems of the past.