It sounds to me like there is still an adjustment problem. Quite a few newbies think they are adjusting correctly, but are not, so please pardon my questions if they seem a bit too obvious.
It is very important to follow the procedure in the factory service manual to a t. Kawasaki used at least three different clutch release systems on the KZ series, so it is crucial to use the procedure for your exact year and model. Basically, the procedure starts by loosening the lever and mid-cable adjusters, then go to the engine. This is where the differences appear. On some models the procedure starts with turning the adjustment screw clockwise until contact, then backing out just a smidge (technical term there). On other models, the screw must be turned anti-clockwise until contact, then in a smidge. It really matters. Then the lock nut is tightened. Next is the mid-cable adjustment, the last is the adjuster at the lever.
If you are certain this has been done correctly and there is still a problem, it could be a badly stretched cable, or some monkey business inside the cover with the release mechanism. Another possibility is that the pressure plate springs are very tired after more than 40 years of being compressed. They are pretty cheap and relatively easy to replace.
She'll Rotella is a good choice, but it is possible that the friction plates have gotten a fatal dose of friction modifiers from the PO using an automotive oil.