ATF oil is hydraulic oil with anti-foam and anti-wear ingredients. If you design a damping fork around it, it is fine. I don;t quite get some people who feel if the fork was designed around ATF, it still is a bad idea to use it. If ATF damps too much for you, by all means, a lighter fork oil (or heavier) is fine...But ATF in a hydraulic cylinder designed for ATF is perfectly acceptable.
Fork oil is similar, except it comes in more viscosity ranges.
ATF takes a real beating in an Automatic transmission. Higher heat, gear mesh, pumping. Fork oil would not stand up to the pounding a transmission gives. Probably is too good for forks....
Using ATF (as recommended by Honda) in my CB360, Fork Oil (as Recommended) in my NH 750. I think my CB260 is a little over damped, so I intent to changed to a lighter fork oil in the future.