Almost any tranny fluid will work in our transmissions. Type F has more friction, which gives faster firmer shifts. That is good to minimize wear of the clutch plates and better performance. Some say F means "friction", others say "Ford". Manufacturers designed slow, slippy shifts, which feel smoother and are what naive consumers think is good. When rebuilding a tranny, smart to install a "shift kit" for firmer shifts and undo the factory's design. Some will even jar your teeth, but intended for race only.
I use ATF+4 because it is better quality. Most or all brands are fully synthetic. Autozone's used to say that. ATF+3 is similar (earlier), but not like Dexron. My 1996 minivan has a 3-spd "torqueflite" tranny, very similar inside to my 60's cars. It specs ATF+3, so should be fine also in my 60's trannys. Many 4-spd trannys in the V-6 minivans were ruined by shops putting Dexron in them, which gave Chrysler an undeserved black eye. Actually kind of their fault because a marketing genius had the owner's manual state that if (then new) ATF fluid wasn't available, one could use Dexron in an emergency, so shops thought "OK and cheaper for us". As I understand, Dexron has foaming agents, with the intent of keeping the inner walls of the tranny clean, but it caused Chrysler's lock-up torque converters to shudder and wear.
