– G –
Graphite – A crystalline form of carbon either natural or synthetic in origin, which is used as a lubricant.
Gravity – See Specific Gravity, API Gravity
Grease – A lubricant composed of an oil or oils thickened with a soap, or other thickener to a solid or semi-solid consistency.
Gum – A rubber-like, sticky deposit black or dark brown in color, which results from the oxidation of lubricating oils in service.