Motor oil starts out amber or light brown but darkens with use, which is actually a good sign. The color change means the oil is doing its job by suspending dirt, combustion byproducts, and metal particles to keep them from settling on engine parts. While dark oil indicates it's working hard, jet-black oil that feels gritty may signal it's time for a change. Modern synthetic oils can stay functional longer even when dark, so color alone isn't the best indicator of oil health.