Just an FYI, ATF is a hydraulic oil. It is lighter weight (thinner) than common hydraulic fluids, but otherwise the same. It is comparable to MIL-H-5606 hydraulic oil, which is equally thin. That oil is the standard for military jet hydraulic systems. The low viscosity is preferred as it thickens up at the low ambient temps that aircraft normally experience at altitude. For industrial and mobile operation, with oil operating temperatures of 110-120 F, 5606 get a bit too thin for some equipment, and is often not liked by many engineers in the industry.
One other point I unearthed (talking to our fluid expert) is that the biggest negative to motor oil in hydraulic systems is the detergent additives, which can cause foaming and oil breakdown in high pressure, high fluid velocity systems. A non-detergent oil is much favored if it is used for hydraulics. But again, if the tranny is operating conservatively, by hydraulic standards, these issues may be minor.
paul