This is the stuff I use:
GL-4 and GL-5 rating. Available anywhere. Like I said, ZERO measurable wear on brass synchros for 30k miles or so. To measure synchro wear, you use a feeler gauge between the synchro ring and the hub. I'd have to look up the clearance, but mine measured no wear. So I reused them with no ill-effect. Shifted like butter.