What fluid for a 4-speed trans?

I'd like to say that when I hurt some synchros ..that it was one of two things...switching to 90w... it the non performance pedal set and very quick shifts with it... but eventually it started grinding here n there and then it didnt slide right into gear, it need more force...you can feel the engagement as where you didnt before. Been 10 yrs like that. ..now I have a kit and will change the synchros and input bearing.
Before you condemn the synchros and the lube. I would make sure you're getting full clutch departure. I've pealed a couple of bell crank arms off of the z-bar, trying to throw fast shifts at the dragstrip. Sometimes the bell crank arms can twist and bend, too, which detracts from efficient t/o bearing travel.