I. Basic Identification Methods
1. Color Observation
Normal new engine oil is light yellow or golden yellow. It is normal for it to gradually turn brown or dark brown after use.
If the engine oil is dark black and turbid (similar to ink) or milky white (possibly due to coolant contamination), it indicates severe deterioration.
2. Texture and Impurities
Dip your finger into the engine oil and rub it. It should be smooth and free of grains. If it feels rough or has metal shavings, it indicates severe contamination.
Drop a drop of engine oil onto a piece of white paper. If the central black area is too large (with a light yellow edge less than 1mm), replace the oil immediately.
II. Causes of Blackening
1. Normal Oxidation and Cleaning
Engine oil naturally turns black due to high-temperature oxidation. Dispersants dissolve engine carbon deposits and sludge, darkening the oil.
If the engine oil turns black rapidly over a short period of time (e.g., after driving a few dozen kilometers), it may be due to a mixture of new oil and residual old oil. 2. Abnormal Contamination Signals
Fuel Issues: Low-quality fuel (high sulfur/olefins) easily forms sludge, accelerating oil blackening.
Impurities: Metal shavings, unburned particles, or dust enter the oil pan, causing discoloration.
Water/Coolant Inclusion: Milky-white oil may indicate water ingress or a coolant leak.
III. Professional Testing Recommendations
1. Laboratory Testing
Acid value, flash point (≥180°C), and oxidation stability are used to determine if oil replacement is necessary.
A particle counter measures impurity levels (NAS 1638 standard).
2. Oil Purifier Treatment
If contamination is detected, a vacuum oil filter can be used to remove water and impurities. However, severe blackening (e.g., with large amounts of metal shavings) requires immediate replacement.
