96 5 speed shifts hard when hot?

I have a 96 Tacoma with a 4cyl. 5 speed with about 180K miles. It seems after about 45 minutes it gets difficult to shift. The clutch works fine and I was wondering if I should change the gear box fluid. Let me back-up, I bought it with 170K and dumped the old fluid from the trans and put in new fluid. I knew this was a no-no on auto trans but is the new fluid to thin and getting hot? Engine does not run hot, but I do live in SW Florida. Anyone ever use Amsoil fluid? Is there a special kind I should use like for racing that may work better than stock? I can get a new 5 speed from a 6 cyl. Will this fit with the 4 cyl I have? Any Ideas would be helpful. Thanks!