Difference Between Oil and Grease

The key difference between oil and grease is that oil has a comparatively low level of viscosity, whereas grease has a very high initial viscosity.

Oil and grease are two types of high viscous substances; these two compounds differ from each other according to the viscosity level. We can easily distinguish oil from a grease depending on the level of viscosity.

1. Overview and Key Difference
2. What is Oil
3. What is Grease
4. Side by Side Comparison – Oil vs Grease in Tabular Form
5. Summary

What is Oil?

Oil is a non-polar substance that is a viscous liquid at normal temperatures. It is both hydrophobic and lipophilic. Generally, an oil contains a high content of carbon and hydrogen, which can make the oil flammable and surface active. Mostly, oils are unsaturated lipids that occur in the liquid state at room temperature.

The origin of an oil can be an animal, vegetable, or petrochemical source. Some oils are volatile whereas others are nonvolatile. We can use oils for food production, as fuel, for medical purposes, for lubrication, and the manufacture of paints, plastics, etc.

Figure 01: Olive Oil

There are two major types of oils as organic oils and mineral oils. Organic oils can be obtained from a large diversity of plants, animals and other organisms. This oil extraction is done through natural metabolic processes. Organic oils can contain components other than lipids, including proteins, waxes and alkaloids. When considering the second type of oils, mineral oils, these oils include crude oil or petroleum and its refined components (collectively these oils are called petrochemicals), etc.

When considering the lubrication property of oils, they are non-polar and do not easily adhere to other substances. Therefore, oils are important as lubricants for various engineering purposes. Moreover, mineral oils are common as machine lubricants than biological oils.

What is Grease?

Grease is a semisolid substance which is useful mainly as a lubricant and is formed as a dispersion of thickening agents in a liquid lubricant. Generally, grease contains a soap emulsified with mineral or vegetable oil. As a common feature of grease, we can observe that it has a very high viscosity. Upon the application of a shear force, the viscosity of grease drops to give an effect of an oil-lubricated bearing that has approximately the same viscosity as the base oil that is used in the grease. We can call this change in the viscosity as shear thinning.

Figure 02: Wheel Bearing Grease

In general, a true grease contains an oil and another fluid lubricant that is mixed with a thickener (e.g. soap), forming a solid or a semisolid. Greases are considered as pseudo-plastic fluids, meaning that the viscosity of the fluid reduces under shear. Moreover, the reduction of shear force with time makes grease thixotropic. However, some greases become more viscous when worked. Often, grease is applied on a surface using a grease gun.

What is the Difference Between Oil and Grease?

Oil and grease are two types of high viscous substances, and the two compounds differ from each other according to the viscosity level. The key difference between oil and grease is that oil has comparatively a low level of viscosity, whereas grease has a very high initial viscosity.

Below is a summary of the difference between oil and grease in tabular form.