Some people have used Amsoil ATF and report no problems. As you have gathered ATF is not recommended nor does Amsoil recommend using thier ATF. Many members have been using Valvoline's semi-synthetic power steering fluid with good results.
ATF and power steering fluid are different in thier make up. From what little I was able to research, power steering fluid is a hydraulic fluid with an anti-foaming additive. Maybe someone knows more and will chime in to enlighten us on power steering fluid vs ATF chemical properties.