You can buy Bar's jack oil with stop leak, which works well (I've used it). It stopped a leak on my jack within a month, and the action is smooth with it. You can use any hydraulic oil, including ATF. If the AFT contains extra seal conditioners, it may help to stop a leak on your jack as well. I understand that this isn't the stated need, just something worth noting. Keep in mind the intrinsic, funny smell of ATF, if it gets on you or all over the jack, then it could follow the jack for a long while. That being said, I think I'd still rather go with ATF in the future since it's there (I do D&Fs) and floor jacks take so little to fill.