If your clutch center is that worn you should also check the clutch basket. The plain steel discs bite into the soft alloy of the clutch center (10) and eventually as you discovered dig a groove which can prevent them from freeing up when the clutch is pulled in. The fiber discs (9) on the other hand tend to wear a groove on either side of the slots in the clutch basket (19) which in turn can cause this problem. Having both worn and I am sure that if the center one is as worn as you say it is, there will be some more damage on the basket too.It was Valvoline bike oil from Advance auto. I switched to cheapest Wallmart auto oil Supertech 20W50.
I have to explain why it helped.
Not because bike oil was bad and auto oil good. I wrote before in another post that my clutch dragging/gear switch problem was (I believe) because steel clutch discs 10 were reinstalled in different places so worn out spots on clutch center 8 didn't match original teeth.
I think that you have to reinstall steel discs in exactly same order and places they were before. When I filled auto oil slippery additives helped steel discs move over worn out spots.
Did I explain it OK? Again: each tooth of each steel disc digs his own hole on clutch center and you better install them back in right place!
Yeah but they have taken the ZDDP out of todays oils and replaced it with moly .In 215,000 miles and 32 years my clutch does not slip with automotive oil. In 1977 there was no such thing as motorcycle oil and Honda recommended 10w40 automotive oil.