I thought one of the others who run a manual transmission might reply. Here's my two-cents'-worth.
The synthetic gear lube won't hurt. When you drain the old lube, check it for metal particles. The synchronizers may have been hashed by somebody who got impatient with the transmission. If this is so, the new lube will put off the inevitible, that is a new or rebuilt transmission.
While you're under the car, you might as well change the lube in the transfer case and differentials at the same time as the transmission.
Don, y'all
.