The Cossack has an oil frame, how much oil went into it?
As far as types of oil it has to be JASO MA2 for use with a wet clutch. That should be right on the front of the bottle.
It has to be flushed out and may require cleaning the plates or replacing them, but not before flushing it out at least a couple of oil changes, I would think the additives the better they are the worse it is, they will likely linger and particularly if there is allot of residual oil in that frame.