The best kept and most closely guarded secret in the United States Army is its publication system. If you obtain TM 9-2320-260-LO and read it will list as the authorized lubricant for transfer cases, differentials and transmissions for the 809 series trucks as GO 80/90 down to 40 degrees Fahrenheit at which point GO 75 is approved for temps down to -65 Fahrenheit. GO products apparently do not become "EP" oils as described elsewhere on this thread and are compatible with brass parts. As the 939 series trucks do not have manual transmissions, the Army has dodged the bullet on transmission synchronizers.