Understanding Oil Color
Color can be used as an indicator of oil health, and in many cases it is a reliable field indicator. However, color alone cannot tell the whole story of the oil’s condition. For a complete understanding of oil condition, it is important to use an appropriate test slate.
The color of lubricating oils can range from transparent to opaque. The color is based on the crude from which it is made, its viscosity, method and degree of treatment during refining, and the amount and types of additives included.
A change in oil color signifies a change in the chemistry of the oil or the presence of contaminants. For example, oil oxidation, mixing two dissimilar types of oil, and carbon insolubles from thermal failure can all darken oil. There is also a possibility that the oil darkening is due to a photochemical reaction from sunlight exposure.
Measuring color is based on a visual comparison of the amount of light transmitted through a defined depth of oil. This can be done with a predefined test method and instrumentation or a subjective view of the oil with reference to a color gauge. In either case, there may be a number of variables to monitor for quality results.
The ASTM D1500-07 test method can be used to compare the color of an oil sample to a glass slide. This test is used in lubricant manufacturing for quality-control purposes. It is performed using a standard light source to match a sample to a glass slide. Values for the glass range from 0.5 to 8.0 in 0.5 increments. If the sample falls between two colors, the higher number is reported. If no color gauge is available, the oil color is compared to a previous sample or a new oil sample.
It should be noted that color is added using dyes in some cases to help identify the type of lubricant, such as red for automatic transmission fluid. Nevertheless, an oil with a specific color should never be assumed to be a certain type of oil. It is also important to remember that there is no particular oil color that would require an oil change.