Possibly ATF is OK - I really don't know. I doubt there would be enough residual ATF oil to impact the performance of the forks int eh short term, but I am not sure how the residual ATF would react with the fork oil (if it reacts at all). After my experiences I would be reluctant to use anything other than fork oil to flush the forks. It is really not that expensive and at least you know you run a MUCH lower risk of longer term problems when using it.