Don't want to sound like I'm contradicting "THE MAN" (NIW) but here's another opinion. My B670 Backhoe and B219 FEL manuals both said to use 30w engine oil or ATF just like yours. When I rebuilt each, I asked the resident hydraulics expert from the plant maintenance department, and he said they will work, but recommended I switch to a quality hydraulic fluid. He said modern hydraulic fluids have better wear resistance, seal life, and anti-corrosion properties than the older recommended fluids. Because I was cleaning and flushing anyway, seemed like a good time to switch over. I have never had any trouble since.
Besides, I now can use the same hydraulic fluid in the tractor, FEL, and Backhoe. Simpler and less to remember, and the last thing I need in my life is more complication.