Which of the following is NOT a common type of lubricant?

Fuel is not considered a common type of lubricant because its primary function is to serve as an energy source for engines and other machinery rather than to reduce friction between moving parts. In contrast, grease, oil, and solid lubricants are specifically designed to minimize friction and wear in mechanical systems.

Grease is a semi-solid lubricant that adheres well to surfaces, providing excellent lubrication in applications where leakage might occur, while oil is a liquid lubricant that flows easily, allowing it to reach tight spaces and provide cooling along with lubrication. Solid lubricants, such as graphite or molybdenum disulfide, provide lubrication in high-temperature or extreme pressure situations where liquid lubricants might fail. They are engineered to minimize wear and facilitate smooth operation within machinery, unlike fuel, which does not serve this purpose.