Your owners manual might specify what to use, but I'd say go with ATF. Why are you looking to change oil? It's not like it gets dirty or full of contaminants. I'd just ensure it was full and operates properly and leave it alone. Filling is a bit messy.
On second thoughts, it might be a good idea to remove your rear seat and rear side panels and inspect the lines that go from the pump to the rams. I had a line go bad and it leaked the ATF all over the place. The only reason I knew I had a problem is there was ATF leaking out of the rear of the passenger door. If I recall, a fitting on the line by the passenger side ram sprang a leak, and the fluid seeped forward and out the door.
I am no expert on this topic upon which I believe you will find a variety of answers ranging from brake fluid to ATF to mineral oil. I personally believe that Clint has offered a great suggestion in that you should check your entire system and start fresh. I have been of the belief that between 1962 - 1967 GM may have used ATF or brake fluid. As you know, ATF has an oily smell compared to the rather pungent smell of brake fluid. When I installed my new top on my '64 CS three years ago, I researched this topic extensively and ended up purging my entire top hydraulics and replacing the lines and cylinders. I used Dextron III ATF and have found my system to be working flawlessly. You could also take Ted's good suggestion and check with a reputable convertible top supplier such as Convertible Top Guys. Good luck with this project.
Late 50's was usually the end of brake fluid in convertible top systems . By 1967 your car should have ATF. Be care full and not fill the pump more then 2/3 full if fluid is
required. Hydro-e-Lectric should have an instruction book available.
