My brother recently was having problems with his gear change sticking and contribute to him and his wife coming of the bike. Both survide with minor injuries.

The issue with the sticky gears seems to be connected to the type of oil that he buys, Is there a particular Brand and grade that is better suited for this model ??
My brother is not mechanicaly minded, so imformation regarding the oils he has tried is limited. I do know he used Motul , 500 i think in the last oil change.
Thank you in avance for any advice offered.

Bongo