If you're referring to GL-4 vs. GL-5 in manual transmissions/transaxle, it is possible to have a fluid that doesn't harm anything, but is TOO slippery for the synchronizers to work correctly. I had this issue on my xB with RP 75W90 GL-4/-5, it was sifficult to shift without grinding gears. Straight GL-4 fixed it.
So I will weigh in because I'm the one I'm that Bill is trying to argue against.
The argument is this:
Mazda recommends GL-4 fluid in the transmission. Some of us use GL-4/GL-5 recommended fluid for better protection/performance, in my case 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 have used the Motul Gear 300 in my 2010 MX-5, which also called for GL-4, for 120K and 8 years without issue.