How motor oil color affects performance. (Hint: It doesn’t).

One of the most resilient motor-oil myths out there is that motor oil that has turned black is unfit for service and must be changed. You can’t go far on the Internet without running into a shadetree mechanic or self-described expert who repeats this myth as proven fact.

Although it makes sense on the surface, 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.

So, what causes oil to turn black? And what gives it its color when its new in the bottle?

Additives Influence Color

Let’s take the second question first. The oil’s additives influence the final formulation’s color. Some oils are naturally lighter and some are naturally darker depending on chemistry. Signature Series Synthetic Motor Oil, for example, has a slight reddish tint due to its additives.

AMSOIL 0W-20 LS-VW Synthetic European Motor Oil (EZT) has a greenish/blue color due to its unique additives that are required by Volkswagen.* Formulators can also add dye to the lubricant to change its color. Transmission fluid offers the perfect example. Red dye is added to prevent confusion with motor oil.

Some companies dye their oil presumably for marketing purposes. Royal Purple* comes to mind. They get a lot of mileage out of the unique color of their oil. LIQUI MOLY* recently introduced its MolyGen* motor oil that’s dyed fluorescent green. I’m not sure why you’d dye an oil the same color as some coolants, but I’ll leave that discussion for another column.

Why Motor Oil Turns Black

Now, let’s look at why motor oil darkens and sometimes turns black. As the oil circulates throughout your engine, it tends to assume the color of whatever it touches. If your engine contains carbon deposits or sludge, which are dark brown or black, the oil will slowly assume that color as its detergents and dispersants clean deposits and hold them in suspension. In this case, dark motor oil is confirmation that it’s cleaning your engine.

Soot also darkens motor oil. While motorists associate soot with diesel engines, gasoline engines also produce soot.

Soot particles range from sub-micron size to 5 microns. Particles that agglomerate, or combine, can exceed 5 microns. By comparison, a human hair is about 70 microns in diameter. Full-flow filters can’t remove soot particles from the oil, which contributes to the oil turning black. 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. If particles do agglomerate, the full-flow filter can capture them.

Some companies dye their oil presumably for marketing purposes.

I should mention that AMSOIL Bypass Filters, which are efficient down to 2 microns, can remove most soot, but they can’t get it all, meaning oil can still appear black using bypass filtration.

Finally, natural heat cycles darken the oil. During your drive to work, your engine reaches normal operating temperature (typically 195°F–220°F [90°C–104°C]), heating the motor oil. Then the oil cools while your car sits in the parking lot.

During lunch, the oil again is heated as you run errands. The process repeats on the way home. And the next day. And the next. That’s what’s meant by “heat cycles.” Specifically, oxidation is causing the oil to darken. 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. Some additives in motor oil are more susceptible to darkening due to heat and oxidation.

How to Know When to Change Motor Oil

Oil analysis is the only surefire 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 dilution and so on. Oil Analyzers INC. offers testing; get more info at oaitesting.com.

Absent oil analysis, it’s best to follow the oil-change recommendations in your vehicle owner’s manual or on the motor oil label.

The recommended service intervals for AMSOIL products are based on extensive testing and backed by thousands of data points spanning decades of real-world use.

When it comes to judging oil condition, trust the data, not your eyes. Otherwise, changing what appears to be worn-out oil could end up wasting time and money.