63 Spd Manual wont shift into First

I have a 63 Valiant with a stock 3 on the tree, and I am having issues getting the the car into first gear. I have had these issues since I got the car a month ago, but it seems like the recent heat has made it worse. I have ATF fluid in the transmission as per the manual, but every shop I have called said to use 80-90w oil, so I am going to switch that asap.
If I start the car in first, its ok, I am just not able to shift back into first, there is a loud grinding and its impossible to get the car in gear unless I turn the car off. The clutch pedal was soft until the very bottom, so I lengthened the adjustable bolt in the linkage and that helped for a while, but it seems like the problem has just come back, and I'm not sure if its the heat or just a coincidence. Could it be the light ATF fluid?
Also, is it true this transmission has no syncho in first?