Hello, I have a 66 st-300 trans . I want to know what the best tranny fluid is to use? It has 8000 miles on it , has not been used since 1970. Also does this tranny have the variable pitch converter in it? Its in a f-85, I have not found any for sale, so if it goes bad it looks like it could be hard to get a new one?

 

I am not an engineer, but was taught that the dexron mercon will pass all previous performance standards...whatever they were. I use the stuff and don't have any issues.

 

Newer Dextron should be backward compatible. Yes, your ST300 (AKA "Jetaway") has a switch pitch converter. You can confirm this if the trans has two wires going to it instead of one. The 64-67 Jetaways were switch pitch, the 68-69s were not.