According to the Indian Institute of Science, Bangalor, oil is the general term that applies to all liquid lubricants, while greases are oils that have been mixed with a thickening agent, which turns them into a semi-solid material. Rather than imparting additional lubrication to the mixture, the thickening agent holds the lubricating oil while reducing its viscosity.
Greases and oils both lubricate similar materials, but some applications favor one type over the other. For example, oil is easy to pour, which makes it convenient to use in car engines. Additionally, as the oil gradually undergoes chemical changes while lubricating the engine and collects bits of debris, it must be replaced periodically. This is easier to accomplish with a liquid that can be drained easily, as opposed to a grease, which does not flow well.
By contrast, greases are better suited for applications dealing with sealed containers or components that are hard to access. For example, bearings and gears frequently rely on grease as a lubricant, rather than oil. Unlike oil, greases do not carry heat away from moving parts and are unsuitable for use as a coolant. Additionally, because grease does not transport heat well, it is used in lower-speed applications than oils are. Eventually, most greases turn into a full liquid or a hard deposit.