Why does engine oil become dark brown or even black over time?


Engine oil becomes a darker brown or even black over time due to various reasons. During daily operation, when the engine reaches its normal operating temperature, the motor oil heats up. And when the vehicle sits in its parking spot, it cools down. 


Some additives in motor oil are also more susceptible to darkening in the presence of heat than others. In addition, normal oxidation can darken oil, too. Over time, the oil’s viscosity or thickness changes, and it also can become contaminated with dust and dirt. This is what causes it to change color and become blackish in appearance.


Thick, dark colored engine oil usually indicates that the oil has been exposed to contaminants such as dust. The darker color oil does not necessarily hamper its efficiency. However, when regular oil changes are neglected, normally free-flowing lubricating oil breaks down, becomes contaminated, and ceases to flow as well.

When you get your scooter or motorcycle, make sure to change your oil every 1,000 to 3,000 kilometers or so.