I've been playing with diesel oil lately and noticed its a few shades darker vs car oil. Could be bias. Also playing with hydraulic oil, and its shades lighter vs car oil. Resisted a taste test but only barely.Years back an older fellow that had worked for Exxon Mobil told me the color is caused by the additives used and even the base oil makeup. I remember growing up new motor oil was dark amber, nowadays it seems to be almost like cooking oil. I must be getting old