Also, Not sure how Motul gets an oil to perform GL4/GL5 work, but nottheless, our MT transmissions require GL4.
GL5 contains Limited slip Friction additives that eat "Yellow" metals like Brass, Bronze etc. I believe the Synchros in our MT have yellow metal.
For that reason, I'd also steer away from the GL5 ratings.
Regular Gear oil will be either 80W90 but if you go with a Synthetic, it will become USUALLY 75W90. Same would apply to other weights. So you are ok with those grades.
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