My 928S4 had a transmission problem that was progressively getting worse. The transmission slipped from 2nd to 3rd gear. After some checks and filling up ATF they problem was not solved and I replaced the ATF. The ATF was brownish and smelling badly. Looks like burned clutches. I did not dare to take the tranny apart, so I let a specialist deal woth it. This is what I got back after it was repaired...

Both K1 and K2 got new friction plates and I think the rear brake band was replaced (not sure).