Why is the car engine oil very black?

[LaPatrick]

Car engine oil turns very black due to the following reasons: 1. Poor fuel quality; 2. The engine operates under high temperature and overload for extended periods; 3. Untimely replacement of the oil filter and air filter. The solutions for very black engine oil are: 1. Replace with suitable fuel; 2. Avoid prolonged high-temperature and overload operation of the engine; 3. Replace the oil filter and air filter promptly. The functions of engine oil include: 1. Reducing friction and high temperature; 2. Cooling and lowering temperature; 3. Cleaning and purifying; 4. Damping and cushioning; 5. Preventing rust and corrosion. Engine oil, also known as engine lubricant, is composed of base oil and additives. The base oil is the main component of lubricating oil, determining its fundamental properties, while additives compensate for and improve the shortcomings of the base oil's performance.

[VonJoanna]

I deal with over a dozen cars daily and find darkened engine oil is extremely common. Fresh oil has a translucent amber color, but gradually darkens and turns black with use. Engine oil acts as a cleaner inside the engine, absorbing metal particles, carbon deposits, and other contaminants. High-temperature operation is like cooking in a pot—the oil naturally oxidizes and changes color. If the oil turns black within days after an oil change, be cautious: it may indicate damaged piston rings allowing fuel to mix with the oil, or failure to replace the oil filter during the last service causing rapid contamination. Engines used primarily for short trips accumulate more carbon deposits, darkening the oil faster. I recommend checking the oil’s diffusion pattern with an oil test strip every 5,000 km—don’t wait for the dashboard warning light to take action.

[DeEmilio]

As a car owner who commutes 50 kilometers daily, I pay special attention to the condition of engine oil. Darkening engine oil isn't always a bad thing; it actually indicates that the cleaning function is working by adsorbing carbon deposits. Last month, when I checked the dipstick myself, I found the oil as dark as soy sauce and immediately went to the repair shop. The mechanic told me: It's a good sign if the oil turns black after six months of normal use, proving the oil is doing its job. However, if you smell a burnt odor or the oil feels sticky, it's a sign of deterioration. Now, during every maintenance session, I extend the time by half an hour to observe the condition of the old oil. If the waste oil drained from the oil pan contains metal powder, a deep engine cleaning is necessary. Regularly changing to the same brand of oil is also crucial, as mixing different formulations can accelerate deterioration.

[DeNolan]

The core reason for engine oil turning black is related to chemistry. I've specifically researched this - when engine oil operates in high-temperature environments between 70-200°C, it undergoes oxidative cracking, causing molecular structure changes that deepen its color. After detergent dispersants capture carbon deposits, they form suspended colloidal particles, naturally turning black. Laboratory data shows: mineral oil begins discoloration at 1,500 km, while full synthetic oil only shows noticeable changes after 3,000 km. Abnormally rapid blackening usually indicates mixing with carbon particles from blow-by gases, suggesting poor piston ring sealing. Using inferior oil additives can actually disrupt formula balance and accelerate sludge formation. Professional oil test strips examining precipitation ring diffusion patterns provide the most accurate assessment.