I come to this thread late, but I recently bought an '85 ZN with a slipping clutch. It was suspected to have the wrong oil in it, too. I did an oil/filter change and most of the slippage went away. I pulled the plates out and scoured them with a brush and some laquer thinner to remove any residual film from the wrong oil. No more slippage.
Some of those "wonder oils" really do coat the metal with a slippery film and it won't just wash off with new oil. It has to be mechanically removed (scrubbed off!).
