2. Color

The color of an oil is evaluated by comparing it to standard glasses. There are several evaluation methods, but ASTM color (NF T 60-104 /ASTM D 1500/ISO 2049) is by far the most widely used. It is obtained by comparing the oil by transparency under a given thickness with one of the 16 standard glasses on the ASTM scale graduated from 0.5 to 0.5, from 0.5 (light) to 8 (dark = red-brown). For colors that are too dark, the sample can be diluted (Dil).

The lighter the color of a base oil, the more refined it is, but additives almost always darken base oils. Some even blacken them completely (graphite, molybdenum disulphide, etc.).

Oil color changes with use. This is obvious for engine oils, which quickly turn black as they become loaded with combustion soot, but it is also true for clear oils (hydraulic fluids, turbine oils, etc.), which become...