I tried synth in my 83 Kz750L. keep in mind that the motor has around 100,000 on it. It worked okay, but I started to get some leakage from the cylinder head and the clutch slipped a bit more than normal.
Other than that it ran fine. I wouldn't see any issues, but if I remember roght the YZF was liquid cooled correct? I'd think cheap old dino oil would work fine