I was recommended by mechanic friends to use GM SyncroMesh Oil on my previous Mitsu Eclipse SyncroMesh Trans.

Since the Rx-8 has a syncro trans, has anyone used or have thoughts on using this trans oil?

any thoughts?

You are in the wrong section first of all, and secondly I probably would think twice about using GM Syncromesh fluid as I am pretty sure it is rather thin, use something like Redline or Royal Purple.

synchro mesh is an additive.

I'm pretty sure nearly all manual trans for the last 30 or 40 years have been synchro-mesh.