What transmission fluid do you use in a 1965 mustang 4 cylinder?

A 1965 Mustang with a 4-cylinder engine likely used Type A Automatic Transmission Fluid if it had an automatic transmission. If it had a manual transmission, it used gear oil, the specific weight of which would depend on the climate and the manufacturer's recommendations (often SAE 90). You'd need to check your owner's manual or a reputable parts supplier using your transmission's identification to be absolutely certain. Type F transmission fluid was also used in some Ford automatic transmissions of that era, so verifying is crucial.