Ok, as much as I have Googled it in the past few days there seems to be no one right answer. ATF, Power Steering Fluid, 10W/40.... What fluid is supposed to be in the system?
Every modern truck I've messed with used engine oil,but, the power steering fluid would depend on the vehicle,not the engine. Good luck.
Should be a simple question wouldn't ya think? I've gone so far as to ask the rather large company that rebuilt one of our steering boxes, and the answer was: 'Uh, well, whatever is supposed to be in there'. I was not too impressed. We've never had anything newer than 2000, but the best guide is the book that came with the truck or better yet a decal on the pump. I've seen the exact same parts on different brands of trucks with different recommendations from the truck manufacturer.
I've seen a lot of them using ATF but I prefer hydraulic oil. ATF doesn't lubricate very well in a pump or gearbox. jr