* Residual Old Oil: The most common reason. It's impossible to completely flush all the old oil out of the engine. Some remains in the galleries, passages, and filter housing. This mixes with the new oil, darkening it.
* Contaminants: Your engine produces various contaminants during operation, including combustion byproducts, metallic wear particles, and fuel residues. Even a short driving period can introduce enough of these contaminants to noticeably darken the new oil.
* Oil Additives: Some oil additives are naturally darker than others. The appearance of the new oil might be darker than expected due to its formulation. Check your oil's specification sheet to see its typical color.
* Type of Oil: Different types of oil have different shades when new. Synthetic oil, for example, may have a slightly darker hue than conventional oil.
* Overfilling: If you overfilled the engine with oil, it may appear darker simply due to the presence of more oil from the bottom of the oil pan, which tends to contain more deposits.
In short, a dark color immediately after an oil change is usually normal and nothing to worry about. However, if the oil continues to darken *extremely* quickly or shows other signs of contamination (like metallic sludge or unusual smells), you should have it checked by a mechanic to rule out any underlying engine problems.
