I used to get Re-refined base oil samples (Used oil re-refined). The color of the base oil is not stable. Color 2.5 becomes 3.5 in two weeks. It further degrades and becomes dark in two months. This is happening in room temperature. Is it because of oxidation? Does oxidation takes place in room temperature when the sample container is closed. The ICP results show similar result that of Virgin base oil. All elements are less than 2 ppm. Can anyone tell me the reason and to stop this degradation?