Mine calls for ATF too. I dont think there was such a thing as "Power steering fluid" in the 70's. I used ATF for ages. I switched to PS fluid for 1 reason: The ATF was not as viscous as PS fluid in the really cold temps and it would groan more, and it would be tougher to turn.

When I replaced it, I bled the whole system out when the pump was off (I replaced it with a newly resealed one that didn't leak). I dont know about mixing the fluids but would advise against.

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Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k