Date registered: Dec 2005
Vehicle: W211 E320 Bluetec R129 SL500 - $1300 car ........ Ford F350 6l
Location: CA Bay Area
Mentioned: 9 Post(s)
Quoted: 1070 Post(s)
ATF is hydraulic fluid as well, but having more detergents.
Older cars have been using ATF in power steering anyway, so don't think you harm anything.
For flushing, I would disconnect return hose from the reservoir (the bigger, softer hose) , put it into empty bottle, fill up the reservoir with proper fluid and start the car for few seconds while moving the steering.
That should suck the fluid from the reservoir and dump it into container.
Will take few refills and few restarts to flush everything.
Calculate 30-50% more fluid then the system capacity.