Appearance of oil can be used as indicator of the health of the oil. The appearance of an oil can range from colorless, transparent, pale yellow, yellow, bright yellow, amber, brown, dark brown, hazy, turbid, opaque, milky, black, etc.
A change in appearance usually indicates either a change in the chemistry of the oil or contamination of the oil.
There are a number of possibilities for the change in apperance of the oil, some of which are highlighted as under :
- Oxidation of the oil
- Mixing of one grade of oil with other
- Oil darkening due to petrochemical reaction
- Ageing of the oil
- Water ingress in the oil
- Soot formation, insolubles, dirt and solid contaminants
- Varnish – or varnish potential – which is a thin insoluble contaminant formed due to depleted additives or by products of oil degradation