As far as color of the oil after a given period of time in use, everyone has mentioned the various factors; engine condition, drive cycles, sludge in a neglected engine being removed, etc. What I haven't heard is that the discoloration is indictive of the contaminents being held in suspension. An oils ability to not loose it's viscosity, as well as it's ability to hold contaminents (chemical, mineral and particulate material) in suspension are the primary factors in determining change intervals. Once the oil is unable to hold contaminents in suspension, as well as maintain it's given viscosity (base and wear additives), the oil ceases to perform it's function as desired. Engine wear then goes through the proverbial roof.
Bill
