Personally, I would not use USED ATF. Used trans fluid contains microparticles of friction materal from the clutches and bands and could cause wear in the IP and injectors.
Oregon Chapter Leader
My friends call me Don.
"I always keep a supply of stimulants handy in case I see a snake, which I also keep handy" WC Fields
RIP Steve Price. A true friend and a tireless supporter of FTE