I wish I still had the link to the Porshe forum I stumbled across where there was a huge discussion on this subject. See, they have a problem where the transmission and axle share the same fluid. One of the GL ratings (5, I believe) is really good for lubricating differential gears because it resists the shear forces, but eats the syncros. The other GL fluid that works really well with the syncros is not good for the diff.
There was one, special, expensive brand name fluid that was close enough to both to work adequately.
The point being that the GL-3, 4, & 5 names are types, not ratings. One is only better then the other depending on application. That quote is very good.
Gear oils are much higher numbers for the same viscosity then motor oils. Dextron was 10w-20 I believe, and Dextron III is 5w-20, and I believe that's similar to a 65w80 gear oil. But I'm pulling these numbers out of fuzzy memory.