Due to my clutch(es) squeaking I have tried about 20 different oils in the last 10 months. Of course my intention was to get rid of the squeaking. I had no problem with slipping.

All my gearboxes on all gassers I had, one GP300/2018, my former EC 300/2019 and my current EC 300/2019 were rattling very loud. Once th eclutch lever was pulled it disappeared. As my KTM's had this also to a certain extent and as all others said they have it too it seems to be proven that this is normal and can not be cured with any kind of gear oil.

However, the best oil to reduce the squeaking was till today the Bel Ray Gearsaver W80. I can't remember who recommended it to me but I think it was swazi_matt.
It performs well, gives good shifting feel. The Motul 10/40 aswell as the 10/30 is equally good, but in my bike the clutch squeaked more with it than with the Bel Ray.

After I have tried so many oils I can say the squeaking has nothing to do with the viscosity, I used ATF which is thin as water when hot and it did not change anything, except that it improved clutch drag when engine was cold or hot.