You MAY declare Mayday and then reduce to PanPan later. One point is crucial: If you need to divert from your clearance due to performance, i.e. you have to follow a CP, then you must declare a Mayday.
Everything else is: Follow your SOP's, follow local regulations (see the info regarding Japan above), but follow as well common sense and handle the situation according your best knowledge and experience.
Finally you are sitting in the aircraft in distress and every controller or later on a laywer cannot follow your thinking and doing at this very Moment you lost some power and Performance.