How gearbox feels is one detail that oil can change.
Especially when engine is really warm.
I like when shifting gear is smooth and easy. Not hard and clunky. How easy the neutral can be found with hot engine and standing still is one safe card to a good oil as I see it.
Hotter running engine need better oil. Cheap oils do not keep the pressure when really hot.
My stock CB750 has Spectro 20W-50 Heavy Duty mineral.
Let's see how it will feel a hot sommer day. And its oil pressure.
My K6 is modified with higher compression, double amount of hp has tried differerent oils. Red Line 20W-50 Esther synth is best so far. Oil pressure that keeps up in a hot engine is vital on a modified engine.
Oil pressure gauge is a must for me since last year. It took me 39 years of owning a CB750 to realize that
My bikes get oils specified for wet clutch. No car oils that have anti friction additions.