I think you should also take into consideration the terrain and weather you are riding in. I would have never thought about it but about 2 months ago i rode for a month straight in a very muddy wooded area with heavy rain fall through out the whole month and when i drained that oil, it was literally dark and muddy which usually doesnt happen when i ride the dry trails for a while. so i now i know to do the oil change faster depending on where im riding.