it has ATF in it for a reason, like bbforks said, it is thinner than normal hydraulic oil and it has gripping qualities. Now before you go throwing in just any ATF it might help to find out the recommended type of ATF it has. Reason i say this is because alot of those older lifts used mercon fluid which is a ford type F fluid i believe, and it is non-synthetic also. You dont want any kind of synthetic based fluid because it will indeed make it slip then. The same thing applies to regular hydraulic oil like AW32 etc, i wouldnt change the type of oil at all if i were you. If you do it will cause bad slipping and then once that gets in the disc its done... you will wind up replacing it.

As for straightening the existing clutch disc? i suppose you can try that but ask yourself why it was warped in the first place? those fingers on the pressure plate should all be even. If any of those fingers are uneven then you have a spring pressure problem and it is probably applying uneven pressure on the disc. This could be why the disc is warped?

with all the time your spending dissassembling and reassembling this you could just try and get a new clutch and pressure plate and rule out those possibilities all-together?

just a couple more things to ponder while scratching your head ;o)