Change oil and filter. Dirty oil has dirt and acids that are in suspension, too fine to be filtered by the filter. Anyway, the filter can't filter out gas and acid, plus the oil loses stability from heat aging.
I have the 2003 results from Motorcycle Consumer News, where they tested 30 oils for heat aging, anti-wear additives, acid neutralizers, and viscosity. I also have the 1994 MCN tests of synthetic vs petroleum vs car vs bike oils. I'll put the results in a separate thread (insert flaming smilie here).