Why Does Motor Oil Turn Black? John Baker | Oct 11, 2017 8:35 AM Motor oil turns black during use for a couple reasons: 1) Heat cycles During your drive to work in the morning, your engine reaches normal operating temperature (typically 195ºF-220ºF), heating the motor oil. Then the oil cools while your car sits in the parking lot. During lunch, the oil again is exposed to heat during your drive to Walmart for butter and shoe laces. The process repeats on the way home. And the next day. And the next. That’s what’s meant by “heat cycles.” The continual exposure to periods of high heat naturally darkens motor oil. Some additives in motor oil are more susceptible to darkening in the presence of heat than others. In addition, normal oxidation can darken oil, too. Oxidation occurs when oxygen molecules interact with oil molecules and cause chemical breakdown, just like how oxygen causes a cut apple to brown or iron to rust. High heat accelerates oxidation. 2)