Motor oil, ATF, AW46, Rotella 15w40, vegetable oil, and water are all hydraulic fluids when used in a hydraulic system.I would not use any type of fluid that is classified as hydraulic fluid. Rotella 15w40 is to be used. Now I have a 448 that is setup for snow blowing only I had to go to 5w40 because I was popping hoses do to the extreme cold.
The root of the word hydraulic is the classical Greek word for water . . . hydor.
The hydro in my GT and the separate loader hydraulic system both use Dexron ATF and have for the past 22 years of service life. Never popped a hose, even at -30° F.