It's fine to use ATF, but I have been told that there is less piston/bore/oring wear with motor oil. An advantage to using ATF is that if it foams because you let the pump "suck" air, it separates out much faster because it has a high amount of anti-foaming agent in it. If you do that with motor oil, it takes quite awhile to separate back out. I prefer the lubricity of motor oil and just take care in fresh hook ups to make sure I don't introduce much air in the system. I guess the Quicksilver Power Steering and Trim Fluid is basically a low viscosity motor oil with good anti-foaming properties.

trim pump
 

i would use 10 w30 motor oil:cool: