This is a very interesting subject. And, like most others very subjective!
I am currently on an ICC (dry clutch) and use very lite weight motor oil in the gear case for 2 years with no issues. For 15 years (add 5-6 more on MX engines previously) I had been running ATF in my RS125 (GP bike engine). The clutch was wet and it never gave me any trouble, even with the tall 1st gear.
No matter the type I still change it after every weekend. The ATF on a wet clutch gets dark and stinks like clutch material, can’t say it broke down at all after 1 weekend. The oil in the dry clutch looks just like it came out of the bottle but I change it anyway.
Just 1 gear related problem in 15 years on that RS, 4th gear went out of it. BUT….that was my own fault. I had multiple ratios for all 6 gears and most of them were purchased used from bike guys. The dogs were questionable yet I talked myself into using it anyway.
Imagine racing Grattan, in a laydown 125, with only 5 of your 6 speeds, having to shift from 3rd to 5th 3 times per lap, for 30 minutes! Tough on the ole forearms for sure, can’t explain it but we ran decent times (about 1.2 sec off our best at the time) and beat some single 250’s winning the Unlimited class. I don’t recommend this practice, if it’s truly broken it could have caused catastrophic damage. In my case it was just worn dogs……and I was pissed…..so I ran it anyway!!!!!
Beware: All the above is just my own personal experience…….as it happened to me……….otherwise known as “hearsay” :)