I'm curious, where are these reviews (do not doubt Redline, just want to make sure we are taking apples)? Isn't GL4 for manual transmissions? The OP never mentioned if he has an ATX or MTX (auto vs manuals use VERY different fluids) but since he questioned Mercon V or Mazda M-5, I'd assume we are talking auto... For the Mazda 5 (all years), Mercon V only goes into your power steering.
Ford owns Mazda (back then). Mazda M-5 is repacked Ford FRN5. See below that the chemical properties of Mercon V is completely different from Ford FRN5 (aka Mazda M-5).
https://www.mazdas247.com/forum/sho...-on-the-Mz5!&p=6341208&viewfull=1#post6341208
Technically, an oil is an oil is an oil. It lubs, protects, keeps temp down, cleans. The difference is some work better (flows better are lower temp, don't get burnt as easily in higher temp, coast/sticks better, has more detergent to stay clean, etc.). You can use most any oil of the same weight in the short term if doing frequent changes. It just might now shift as smoothly/quickly as it could/should and bring on added wear in the long term.