Motul is a good oil but it does not mask any faults like Redline can. A slightly crunchy synchro will usually be helped by Redline. I wouldn't spend $800+ getting the synchro changed without trying $200 worth of oil first.
It does not need to be changed every 10,000km, I'm not convinced it causes all the grief it gets blamed for either.
Driving with a crunchy synchro will eventually damage the gear as the synchro dogs are part of the gear. Once the points of them are burred over it's fugged.