[strat61caster]

I have Redline MT90 75w-90 GL4 right now, about 8k miles and I can't point to a difference between it and Motul Gear 300 or the OE fill (both for 30k miles). I'd bet even with the GL5 (which iirc is acceptable per the users manual, please check it yourself though) there shouldn't be an issue if it has enough fluid in the trans.

There may be an issue with the clutch install causing the bad shifting, even great shops make mistakes, talk to them, have them drive it, discuss a fluid change and if that doesn't work have them check their work and inspect the clutch install, there should be minimal or no cost to you, specialty shops live and die by their reputation. Best of luck to you.