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Black oil does not always indicate lack of maintenance. Fresh oil can turn black in a few hundred miles. Carbed engines such as the GZ are of an older design that can and will do that. Newer FI engines not so much.
When oil changes color it is an indication that the oil is doing it's job and floating unwanted deposits to the filter. The color change is not written in stone but a pretty good rule of thumb. If in doubt, change the oil. As always, we all like that fresh oil color in the sight glass.
You may have a situation where the oil is clean looking but the tranny is getting harder to shift. This may indicate the oil is loosing it's properties and needs to be changed after all other factors are figured into the equation.
Oil is cheap and the life line of any engine. We had Shell Rotella 15-40 in my Vstrom 650 and Lynda's Suzuki M-50. Both changed color very quickly but maintained shifting smoothness for the full distance between changes with very little need of a top up. Rotella is used by many bike riders. It is certified for wet clutches.
Oddly enough my experience with the Vstrom and Amsoil 10-40 MC oil was not as pleasant as Rotella. Louder shifting noises, more frequent top ups and the initial cost were my reasons for changing to Rotella.
As a side note, my Ranger 4.0 v6 was okay with Rotella 15-40 but it really likes Rotella 5-40 Synthetic. I doubt I would use it in a bike though. I don't think it is needed.
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