I've followed the thread about this issue on the Miata Forum, and despite Lee Walter's recommendation, I'm still confused about the use of the Motul product. Recommending it for a car that is tracked but not for one that is street driven... Do you mean it would be of no benefit for a street car, or just superfluous? And the rating of GL4 vs GL5... From the Rymax website, their explanation of the difference between the two ratings would imply that the GL5 will cause harm to the synchros...
The main difference between GL-4 and GL-5 gear oils is the amount of EP additives. Sulphur/Phosphorus containing products are used as EP-additive. This additive has the purpose to prevent the occurrence of micro-welds on the gear flanks at the local high temperatures which prevail in EP circumstances (temperatures well in excess of 800℃!) GL-5 has roughly twice the amount of EP additives compared to GL-4, which is why it is often used in high-pressure circumstances such as in a front axle and rear axle differential.
Sulphur/Phosphorus additives however have an unfavourable property: they can react aggressively towards bronze and copper. This can be disastrous for the synchromesh rings of a gearbox. Therefore it is not recommended to use GL-5 in a gearbox unless the manufacturer allows this.
Having suffered a gearbox failure on an ND with about 20 track days over a 3-year period, I'm faced with a dilemma. Not track the car at all, or do something to mitigate the likelihood of having to replace the transmission in another 2 years - at my own expense. It is good to see Lee Walter and hopefully others exploring alternatives, but you remember what Ronald Reagan said back in the day.