oils break down. the long strands of molecules (ie the chains that make up the viscosity index) break and the VI goes down. They also absorb acids and moisture from the crankcase as well as byproducts of combustion. Sort of like your liver, your filter does its best to filter particulate but it cannot fix old thin oil. Your filters are probably cleaner that you think when you pitch them of particulates, and the color is black because of the carbon the oil picks up. Pull apart a Nissan V6 or a Stratus V6 with the PCV issue and you'll see what lack of oil conditioning/changing does to a motor...GUNK!
