The owners manual says not to use regular power steering fluid but doesn't say what should be used.
Today, the 15W-40 oil is used, no idea what your unit requires for both power steering & hydraulic fans, but the mentioned oil is what I would use, NOT ATF nor 10W hydraulic.
FWIW
Oil requirements have changed in the past because of seal compatibility. As oils change specifications, they may soften or swell seals if the oil is not compatible. The manufacturer will change the requirement as failures occur.
I have seen ATF (type A, suffix A) used in place of aircraft hydraulic fluid (on aircraft) even though it looks but is not quite the same with additives specific to automatic transmissions added.
Best to find out what was OEM and go from there. The 15W-40 that Dave recommends may be the ticket for your earlier rig but nice to make sure and double check. Another possible option if your seals are leaking would be a 15W-40 high mileage oil. They have an extra additive that will soften and cause the seals to swell very slightly and may dry up a small seep/leak.
Pierce
My '91 uses 15-40wt motor oil but I have an Unihome chassis. Your chassis (Oshkosh?) might be different.
Our 91GV with the Oshkosh chassis and the 300 horse Cat diesel uses (dexron III ATF I think). Look on your dip stick for reservoir level and see what color it is. It will be reddish color if ATF and oily looking if not. If you need a good number for it let me know and I will look it up in my manual. Have a great day ---- Fritz
Dexron 111 is the stuff, Red Dyed Steering Fluid, I have just poured 25 litres into mine, It takes just under 5 litres, Both reservoirs, One front and one back, Or Tanks,