Dexron II or III. I use Amsoil ATF full synthetic transmission fluid in my 63. Transmission runs and shifts much better than when I used Penzoil Dexron II and III ATF.
In Sweden the two companies with the most experience of spare parts for classic Cadillacs recommend GULF ATF Type A Suffix A #254163GU01 for Hydra-Matic (Jetaway). Reason being it's thicker than ATF oils for modern gearboxes. Apparently this Gulf oil has the specifications as per original Cadillac specifications. I haven't checked this myself, but I do trust the people stating it.