I have heard that switching my 6MT fluid to AMSOIL 75w-90 is extremely hard on the syncros(what are syncros and what do they do). I am planning on changing the Motor Oil to AMSOIL 5w-30, and both the 6MT and Diff to AMSOIL as well. Will switching the MT fluid hurt the syncros?

 

6MT gear box calls for GL-4 fluids.. do not use GL-5 as the extra extreme wear additive will corrode the yellow metals. Stick with GL-4 for the transmission. GL-5 is fine for rear differential

 

If you want to use synth tranny oil, use Redline MTL 70-80w. Even though its synth oil, its GL4.

 

I switched to Amsoil Series 2000 75W-90 at around 3000 miles to improve shifting ease. I am extremely pleased with the results and the fact that I could use one oil in the 6MT and rear diff, rather than mixing, as Redline users have.
Although the API specs matched Nissan's requirements, I called Amsoil's techs, who reviewed the requirements and agreed that it was an excellent choice.

Amsoil has changed their gear lube offerings since and no longer offers a lube that meets the GL-4 rating for anything but rear diff use. Unless you could locate some of the older Series 2000 lube, it appears like a different brand will have to be used in the 6MT.