Yellow metal = brass, muntz, and certain copper alloys. Brass is what is used on the synchronizers in the Miata.
You need to understand that GL-4 and GL-5 are GEAR OIL specifications and not TRANSMISSION OIL specifications. GL-4 oils are meant to have a lesser degree of sulphur and phosphorus additives than GL-5, as well as a different package of friction modifiers.
Here is why it matters:
GL-5 is meant for extreme pressure and extreme temperature protection. When the sulphur/phosphorus additives contact high pressure and high temps they create a hard coating on any surface they are touching. This is typically a good thing. When the gears this coating is on experience friction between themselves, instead of the gears being slowly worn away, this coating is worn away. Then more of the additives are deposited restoring this coating and re-adding protection from friction and wear. Differentials and transmissions use hard steel gears, so when the friction wears off this coating, it doesn't damage the gears themselves.
However our transmissions have brass synchronizers. These are softer than the coating provided by the additives. When the coating is worn off by friction, it takes small amounts of the softer brass along with it. GL-5 also contains a different type and level of friction modifiers. These prevent the synchonizers from releasing and spinning freely resulting in "notchy" and hard shifts, or grinding if they are really bad.
When changing from one GL-5 to another GL-5 you are going to experience the same result as changing your engine oil. For a little bit while the fresh additive package does it's work of cleaning the old gears, the different(between brands) friction modifiers will help or solve some shift problems, and fresh base oil re-lubricates all the moving parts better than the old. However you are still doing damage due to the different concentration of sulphur/phosphorus and different types of friction modifiers.
To provide the best shifts, and the longest transmission life, you need an oil that provides three things:
1) Corrosion and pressure protection for the steel gears
2) Corrosion and pressure protection for "yellow metal" synchronizers
3) An additive package that allows the synchonizers to slip properly
With the NA, NB, and NC 5 and 6-speed transmission this is going to be GL-4 rated gear oil due to the yellow metal. The differential is void of yellow metals, therefore either GL-4 or GL-5 can be used, however due to the higher pressure protection GL-5 rated oils are recommended for the differential.
Here comes the hard part. Many manufacturers no longer make GL-4 gear oil because GL-5 is superior to it in the proper application(read: applications lacking in yellow metals). This is why many forum members have gone to the Motorcraft and Redline series because they are specifically designed for "synchromesh" transmissions and carry a GL-4 rating.
Think of your transmission as a wall. The steel gears are drywall, the synchronizers are the paper coating on the actual plaster. Now you want to hang a hook on the wall, you have two options: 3M Command adhesive, and 3M grey double-sided tape(the kind they use to attach spoilers, vent visors, etc. that's a #$%& to get off).
The GL-4 is the Command, the GL-5 is the grey double-sided tape from hell.
When you violently remove the Command tape, it comes off clean without any real damage to the paper. When you violently remove the grey stuff, it tears off some of the paper, but leaves the plaster drywall intact. This is what is happening to the gears and synchros in your transmission, just on a much slower scale.
