Transmission fluids such as Amsoil Synthetic ATF state that it is compatible with conventional ATF fluids. You have to check on the transmission fluid labels. Automatics require Mercon V transmission fluid so therefore mixing Valvoline Mercon V fluid with Amsoil ATF will not cause a problem, though it will dilute the synthetic properties of Amsoil which would be the superior fluid. I'm not sure what fluid a 96 5speed takes, but I'm guessing that your manual either calls for Mobil 1 ATF or Mercon/Mercon V. Check your manual to find out what fluid your transmission takes and look on the bottles to see if they are interchangeable. As far as I've seen, most transmission fluids are compatible with one another.
Quote from Amsoil's website:
"AMSOIL Synthetic Universal Automatic Transmission Fluid is completely compatible with other synthetic and petroleum automatic transmission fluids."