08-04-14, 01:32 PM
There's nothing wrong with using fully-synthetic oil in the FZS600, so long as it's suitable. It's just more expensive then the engine needs (though you can stretch the oil change interval). If it says motorcycle oil it should be fine. The oils with friction modifiers (usually molybdenum) are the ones to avoid, they usually brag about fuel economy on the carton.
If you have got moly on the plates, you can supposedly clean them up by dismantling and giving each a good scrub with paraffin.
Other possibilties causing slip are a mistake on assembly or perhaps the adjuster in the sprocket cover being wound in too tight, though I'm not sure how that could happen accidentally.