I am aware of the recommended viscosity.
GL-5 is the required service rating.
I believe the viscosity is of less importance.
Moly added to both transmission and FD oils reduces friction dramatically. Your transmission will actually run cooler, shifting will be improved and friction surface wear diminished.
I like the 75w-140 synthetic because it has a more robust add package, better copper corrosion test results and doesn't foam. Also, it doesn't shear as quickly as non-synthetics.
But, that's me...use whatever you think is the best.