Why motor oil turns black

Here’s why motor oil turns black

Soot from incomplete combustion turns motor oil black

All internal combustion engines create soot as the fuel is burned. Some of that soot makes its way into the crankcase due to blow-by. Soot creation is highest during cold starts due to partial combustion caused by the cold metal “quenching” effect. But even after the engine is warmed up, combustion isn’t always 100% efficient because the air/fuel mixture is never 100% accurate.

The computer tries to calculate the exact air/fuel mixture and it could easily do that if your engine ran at a set speed, set torque, at the same barometric pressure, and the same barometric pressure and ambient temperature. But that never happens in the real world. Instead, the computer reads the oxygen sensor data to see how well it did just a few seconds ago and tries to adjust the air/fuel mixture to correct what it got wrong. So the air/fuel mixture is constantly changing and a portion of that fuel creates soot.

Heat Cycles

One of the primary reasons why motor oil turns black is due to heat cycles. During normal operation, your engine heats up the motor oil to its operating temperature, which is typically between 195°F and 220°F. As the oil heats up, it naturally darkens in color. When the engine is turned off, the oil cools down, and the process repeats itself when the engine is started again. These heat cycles continually expose the oil to high temperatures, which causes it to darken over time.

Additives

Some additives in motor oil are more susceptible to darkening in the presence of heat than others. For example, detergents and dispersants in the oil are designed to clean the engine and carry particulates to the oil filter. As they do their job, they can cause the oil to darken in color.

Deposit removal

The motor oil comes into contact with nearly everything inside the engine during operation, including deposits that may have formed on engine parts. Since carbon deposits are usually black in color, it won’t take long for the oil to turn black if there are deposits inside the engine.

Old oil left in the engine and crankcase turns the new oil black

You never get all the old oil out of your engine during an oil change so the old oil darkens the new oil

Oil color is NOT an indication of its condition

The ONLY way to determine an oil’s condition is through laboratory testing. Color is never an indication of its condition. If any shop personnel show you black oil and tell you it’s time to change your oil, it’s time to find a new shop because this one is bullshitting you.

This motor oil color chart is complete bullshit. There is no scientific evidence that shows any correlation between oil color and oil condition. This chart is fraudulent and is designed to scare you into changing your oil before it’s due

©, 2023 Rick Muscoplat