How Darken Motor Oil Color Affects Performance

A darken color of motor oil is not necessarily an indication of a needed oil change.

Motor oil myth: once motor oil has turned black, it’s unfit for service and must be changed. Oil color is not an indication of condition; oil that has turned black may continue to provide good protection and performance. Oil analysis is the only way to tell for sure if the oil has reached the end of its service life.

What Gives New Motor Oil its Color

Some oils are naturally lighter and also darker in color depending on chemistry. Amsoil Signature Series synthetic motor oil has a slight reddish color tint due to additives.

Amsoil 0W20 LS-VW Synthetic European Oil (EZT) has a greenish/blue color due to its unique additives which are required by Volkswagen vehicles. Red dye is added to transmission fluid to prevent confusion with motor oil.

Why Does Motor Oil Darken?

As the motor oil circulates through the engine, it assumes the color of whatever it touches: carbon deposits or sludge. These will be brown or black which the oil will assume its color. The detergents and dispersants in the oil will clean these deposits thus turn the oil a darken color. A darken motor oil confirms the oil is doing its job.

Soot Also Darkens Motor Oil

Motorists associate soot with diesel engines; however, gasoline engines also produce soot.

Soot particles range from sub-micron to 5 microns. A human hair is 70 microns in diameter. Full flow oil filers can’t remove soot particles from the oil which contributes to the dark color. However, soot is too small to harm the engine since the oil’s dispersants will hold soot in suspension and prevent it from adhering to metal surfaces.

Natural heat cycles darken the oil. On your way to work, your engine reaches normal operating temperatures (195-220 degrees F) heating the motor oil. When you’re at work, the motor oil cools. This process repeats itself on the way home from work, the oil cools overnight. This happens day in day out and this is called “heat cycles” and this invites oxidation. Oxidation occurs when oxygen molecules interact with oil molecules and cause chemical breakdown. Some additives in motor oils are more susceptible to darkening due to heat and oxidation.

Oil Analysis

Oil analysis is the only sure-fire way to determine if the oil has reached the end of its service life. Chemically analyzing an oil sample reveals the condition of the oil, the presence of contaminants, fuel dilutions and other analysis. Go to for further information.

If oil analysis is not your thing, it’s best to follow oil-change recommendations in your vehicle owner’s manual. The recommended service interval for AMSOIL products are based on extensive testing and backed by thousands of data points spanning years of real-world use.