GL-5 gear oil has oodles of E.P. (extreme pressure) additives. These additives are corrosive to soft metals such as brass, bronze, and some aluminum. Causing your oil to literally eat your syncros.
GL-5 contains the most E.P. additives followed by GL-4 with 30-50% less and then GL-3 with 25-40% less then GL-4.
Your best bet is to use factory specified GL-3 gear oil - weight is not very significant. Use lighter in colder weather heavier in hotter weather if you are picky about it.
Or 30 weight motor oil - The E.P. quantities vary GREATLY, best to stick cheap, less chance of additives.
Or Manual transmission specific oil such as Penzoil syncromesh or Redline MTL - They are rated at the lower end of GL-4 for E.P. and most have friction modifiers that produce the correct coefficients of friction for syncros.
I hope this was helpful
Edit: Some definitions
API Category GL-3 designates the type of service characteristic for manual transmissions and spiral-bevel axles operating under mild to moderate to severe conditions of speed and load. These service conditions require a lubricant having load-carrying capacities greater than those that will satisfy APL GL-1 service, but below the requirements of lubricants satisfying the API GL-4 service.
API Category GL-4 designates the type of service characteristic for spiral-bevel and hypoid gears in automotive axles operated under moderate speeds and loads. These oils may be used in selected manual transmission and transaxle applications.
API Category GL-5 designates the type of service characteristic of gears, particularly hypoids in automotive axles under high-speed and/or low-speed, high-torque conditions.