There is some debate on fluids for the most recent generations of MX-5.
Yes, Mazda recommends GL-4 fluid in the transmission. However some of us use GL-4/GL-5 recommended fluid for better protection/performance, specifically the Motul Gear 300, and a lot of others think we're wrong. I am of the opinion that the quality of the fluid means more than rating.
I used 75W90 Motul Gear 300 in my 2010 MX-5, which also called for GL-4, for 120K and 8 years without any issues other than a slight difficult 1-2 shift on cold mornings until the transmission came up to temperature. When I changed it the fluid coming out looked as good as the new fluid going in. This was in a modified car that saw a lot of hard track and autocross use without a single issue.
The Gear 300 is a GL-4/GL-5 rated fluid.
https://d23zpyj32c5wn3.cloudfront.n...106/Gear_20300_2075W90_GB.pdf?1492016023
Some of you may not know but the 2016+ MX-5 transmission hasn't proven to be the most reliable. Especially when used in autocross or track use. I now have a 2019 MX-5 and am concerned about the reliability of the transmission especially it is modified and will see autocross and track use.
As you probably know the MX-5 is a very popular car to use in motorsports. As such they have seen a lot of transmission issues. The MX-5 Cup Cars have switched to an aftermarket sequential transmission to solve their issues. The rest of us aren't so lucky.
Walter Motorsport, an MX-5 race shop has had several of these transmissions apart and has documented what they call a "fundamentally flawed transmission with cheap bearings and poor tolerances." They also state that the factory fluid should not be used as it is the wrong formulation.
They are recommending either the Motul Gear Competition 75W140 or the Lucas Oil 75W140.
https://d23zpyj32c5wn3.cloudfront.n...20Competition_2075W140_GB.pdf?1492016023
https://lucasoil.com/pdf/TDS_SAE-75W-140-GO.pdf
So while I understand that for the majority of seniors who Sunday cruise their MX-5's the best recommendation is to use the recommended GL-4 oil. Most people use the Motorcraft GL-4.
However, there are those of us that need better fluid and make our own decisions about what to use and that starts with a quality fluid over a rating.