Personally I think every manual trans fluid should be changed at least every 40k miles, or more if you agressivly shift. By using synthetic fluid its like doing 2 fluid changes because synthetic has more detergents to clean the transmission than normal cheap stuff.
Redline fluid is the best, along with royal purple. But you can also use mobile one or castrol. Whatever you want, the redline /royal purple is about 8.00 a quart. the mobile one is about 5. It uses about 4.5 quarts to change the fluid.