Why motor oil turns black — Ricks Free Auto Repair Advice Ricks Free Auto Repair Advice

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.

All oil is exposed to heat and air and that causes the additives and oil to oxidize. Oxidized oil and oxidized additives can cause oil to change from a honey color to dark brown.

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

©, 2023 Rick Muscoplat

Posted on by Rick Muscoplat


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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

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.

Motor oil color chart

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 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 at the same barometric pressure and ambient temperature. But that never happens in the real world. Instead, the computer is reading 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.

Oxidation creates discoloration that turns oil black