Pretty well thought out.
The GL-4 or GL-5 indicates the type of additive package in the oil.
GL-4 additive is anti-wear, sometimes there would be an AW in the nomenclature of the oil. It uses molecules that attach to the surfaces. In the beginning these Anti-Wear additives were made from tallow.
GL-5 was developed for the sliding friction that came into being with the invention of hypoid gears, sometime in the '30s, IIRC. These additives microscopically attack the surface of the gears and create an oxide layer. Under high loads, when the oil film is ruptured, the oxide layers on the gears prevent metal-to-metal contact and prevent the contact welding that would occur otherwise. The oxide layers are the last line of defense.
The chemically active additive packages work more aggressively on copper alloys, like the parts of the synchronizers made of brass or bronze. They also prevent the synchronizers from doing their job, which involves a certain amount of friction needed to match speeds between driving and driven parts. So use of the GL-5 oil can cause hard shifting.
There are several manufacturers of manual transmission lubricants that are GL-4. Redline's MTL is a prime example, but there are quite a few others. The problem of high viscosity on cold startup is solved by using a synthetic that has a much flatter viscosity vs. temperature curve. My friend who was an EMT in a high mountain town did that 30 years ago, although at that time there were only a handful of synthetic gearlubes generally used in the Arctic regions. But when he got a call when the temperature were 30 below, his 4wd was ready to go and there was actually lubricant flowing in the transmission and differentials.
One last thing.....Viscosity numbers assigned by the SAE are NOT the same for motor oils and gear oils. They were assigned different ranges of numbers to avoid confusion. So, 90 gear oil is about the same viscosity as 40 motor oil. If you already knew this, you're ahead of the game.
R.