Proper Transmission fluid for '56 Buick Special

[Guest loalre]

I just bought my first classic car, a '56 Buick Special and am trying to figure out proper transmission fluid. I am not a car expert by any means. Know just enough to be dangerous.

I want to top off the transmission fluid and called the owner I bought it from to ask him what he used. He told me he had the fluid changed by a shop and thought they used Dexron 5, but wasn't sure. In a search I came across a thread here that suggested Dexron 3 only for Dynaflows, don't use Dexron 4 or 5. Should I be worried? Or just, when I get a chance, have the fluid changed to the proper Dexron 3 to be on the safe side?:confused:

[JohnD1956]

Last I knew it should be okay regardless of using it in a Dynaflow. However, if you are uncomfortable with this D5, know that there is a drain plug on the transmission pan just like on the oil pan, so it is relatively easy to change the fluid in the pan. What's difficult is this will not drain the fluid in the reaches of the transmission ( the Torque Converter and the Tranny Cooler in the bottom of the radiator) , so just draining the pan and refilling will still result in contaminating the new fluid with some of what's already in there.

What did the internet search say about the 4 & 5 that makes it uncompatible?