Dapepper is pretty close. The New Venture rebuild manual calls for 75w90 synthetic gear oil. They recommend either castrol, mopar, or GM Goodwrench.
They claim that anything else will cause faster wear on the carbon composite syncros. How true that is, I don't know, but i'm usually the guy that goes around screaming "do it to factory".
Here is the exact excerpt in case you want part numbers
75W-90 Castrol Syntorque Synthetic
Mopar Synthetic Manual Transmission Lubricant Mopar P/N 4874459 (Material Specification 9070)
GM Goodwrench® Synthetic Manual Transmission Fluid (GM Part No. U.S. 12346190, in Canada 10953477)
