When I switched the gear oil in my Mexican Beetle (at 10,000km) from whatever the factory used to Amsoil synthetic 75W-90, I noticed a significant improvement in the shifting. I'm sold on synthetic gear oil.
If you look at the fine print, you will see the API ratings. The one that is common today is GL-5. Over the previous GL-4, the GL-5 oil has a better extreme pressure rating. IMO you must have this for any high performance gearbox that sees higher than stock loading.
Every time your swing axle trans is out, pull out the side gears from the diff and inspect the gear teeth. Note the wear on the teeth that was caused by previous inferior gear oil. I've seen plenty of side gears with teeth that were badly worn out.