I know little about transmission oils, but would nonetheless suggest a change every 30,000-40,000 miles, or two years.
As to the question of dino vs. synthetic, all I can say is from my experience, which is that I put Amsoil's FGR, a 75w-90 GL-5 gear lube that is MT-1 rated, in a '93 Toyota, for which the manual spec's any of GL-3, GL-4, or GL-5, and it shifts perfectly. A different oil may be better; I don't know. But I'm happy with what I have.