What Frank Adkins says, be sure to get a GL-4 rated manual transmission fluid which is not the same as a GL-5 rated differential fluid.

Also, one can compare the operational viscosities of the various commonly available lubes.

Notice how the ATF and Synchormesh are close in terms of thickness...

PENNZOIL MULTI-VEHICLE ATF: Viscosity 33.82 @ 104(F)

PENNZOIL SYNCHROMESH FLUID: Viscosity 41.60 @ 104(F)

Now notice how the 75W-90 GL-4 is over twice the thickness...

PENNZOIL SYNTHETIC SAE 75W-90 GL-4: Viscosity 108.00 @ 104(F)

I'd go with a good 75W-90 GL-4