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Oil and grease are two very familiar terms, but most people aren’t clear on exactly what the difference is… unless they happen to work in lubrication, that is! There are some situations where an oil is the better choice, and some when a grease works better. So, what is the difference between an oil and a grease? Very simply:

  • Greases are usually just oils with thickener added.
  • At room temperature, greases are usually solid, while oils are usually liquid.
  • All oils can be turned into greases, but not all greases come from oils.
  • Greases are typically only used on machinery, tools, or equipment, while oils have a multitude of other, non-industrial uses.

thick grease

Grease is just oil with thickener added.


So what is this thickener? It can be a few different things: one of a few kinds of soaps, a type of clay called bentonite, or a mineral called molybdenum are the most common. Most of the greases you encounter will use a complex soap, and of those, the soap they use is most likely lithium.

The chart below offers several examples.

Table of Grease Thickeners and their Characteristics

Grease Thickener Appearance Shear Stability Pumpability Heat Resistance Water Resistance
Calcium Buttery Good Fair Fair Excellent
Sodium Fibrous Fair Poor Good to Excellent Poor
Barium Fibrous Good Poor Excellent Excellent
Lithium 12 OH Stearate Buttery Excellent Good to Excellent Good to Excellent Excellent
Lithium Complex Buttery Excellent Good to Excellent Excellent Excellent
Calcium Complex Buttery to Grainy Good Fair Good Good to Excellent
Aluminum Complex Buttery to Grainy Good to Excellent Good Excellent Excellent
Clay (Bentonite) Buttery Good Good Excellent Excellent
Polyurea Buttery Good Good Excellent Excellent
Calcium Sulfonate Buttery to Grainy Good Good Excellent Excellent

How is grease made?

Grease is oil stored in a fibrous network that functions like a sponge. This fibrous network is the thickener mentioned above. The molecules that make up the thickener will align themselves in such a way that they create a sort of structure with spaces in between, and the oil particles will then fit into those spaces. A grease’s lubricating properties come from the oil that is contained in this network. Other properties are determined by the type and quality of the thickener.

The network containing the oil provides other qualities, such as:

  • Consistency: how thick the grease is.
  • Dropping point: the temperature at which the oil begins to separate from the thickener.
  • Water resistance: washout and sprayoff tests measure how long a grease stays on under water and when sprayed with a high-pressure hose.
  • Base oil viscosity: the stickiness of the oil used to make the particular grease.
  • Load carrying ability: how much pressure a grease can tolerate and still perform well.
  • Shear stability: how well the grease maintains its consistency when worked repeatedly and quickly.
  • Compatibility: it’s very important to determine whether different greases used in the same place will cause each other to lose one or more of their properties. Consult a miscibility chart from your manufacturer.
  • Pumpability: How easy is it to transport the grease via a pump?
  • Oil separation: Some amount of oil must separate from the grease in order to be effective.

Food-grade lubricants

Some greases need to be food safe, or food-grade (the terms mean the same thing). This means that if they accidentally end up in an items that is meant to be consumed by people or animals, they will not cause any harm, as long as they are found below a specific concentration. (Note: just because a grease is labeled ‘Food Safe’ doesn’t mean you can cook with it. You can’t!) Interflon makes several food-safe lubricants. Read more in this post called “What is a food-grade lubricant?”

The NLGI

Grease is an extremely important part of our modern, industrialized world… so much so that it even has its own institute, where people do nothing but think about grease all day long! It’s called the National Lubricating Grease Institute (NLGI).

The NLGI offers a scale for consistency. This rates greases based on their relative firmness, from 000 (fluid, like cooking oil) to 6 (very hard, like cheddar cheese). Again, your choice of grease is going to be determined by the type of application you’re using it for. Very fluid greases can be used in low-speed applications where there is no danger of leakage. Harder greases can be used in high-speed applications.

For more information on the NLGI, visit their website. To learn more about consistency numbers, see this Wikipedia page.

When is it better to use grease instead of oil?

The answer to this question depends completely on your application. It’s usually better to use grease:

  • Where leakage and drippage is present
  • In hard-to-reach places where lubricant circulation is impractical
  • Where sealing is required in a high-contaminant environment (i.e. water and particles)
  • To protect metal surfaces from rust and corrosion
  • To lubricate machines that are operated intermittently
  • To suspend solid additives during slow-speed, high-load sliding conditions
  • For use in sealed-for-life applications such as electric motors
  • To lubricate under extreme or special operating conditions
  • To lubricate badly worn machines
  • Where noise reduction is important

When is it better to use oils instead of greases?

Oils have cooling properties. Oil transfers heat away into a larger body of oil, which can then be pumped through a heat exchanger. So, if the application in question generates a lot of heat, chances are you’ll want to use oil as a coolant as well as a lubricant.

Oils are also used when greases would be harder or more impractical to apply. For example, gun owners typically apply an oil to their firearms after cleaning, because it wipes down smoothly and penetrates better into the pores of the metal, and it won’t make a mess the way grease will. However, if a gun is going to be stored for a very long time, it may be packed in grease to prevent rust or wear from taking place.

In general, oil can be used in any situation where grease isn’t a good idea.

Trivia

One of the most common greases for weapons storage is called Cosmoline. During WWII, entire tanks would be smeared with Cosmoline to protect them during sea voyages. U.S. Coast Guardsmen manning coastal batteries were sometimes called “Cosmoliners” because they spent so much time wiping down their big guns with Cosmoline.

oil drops

Can you mix greases?

The term for mixing greases or oils is miscibility. Some greases and oils are not miscible, because when they come into contact with each other, they can lose one or more of the characteristics that makes them useful. Interflon provides a miscibility chart that allows you to easily determine the miscibility of our various products (click link to view or download PDF).

Did You Know?

Greases are said to be thixotropic. This means they’re thick under static conditions (e.g. when they’re just sitting in a container) but will flow when used (e.g. when they have been applied to a high-speed bearing) over time.

The opposite of thixotropic is rheopectic. This refers to something that goes from a liquid to a solid when shaken. This is pretty rare, but it’s still a good Scrabble word, so you’re welcome.

References

For further reading, check out these links below!

http://www.rbibearing.com/technical/pdf/0405.pdf

http://www.differencebetween.net/object/difference-between-oil-and-grease/

http://www.howstuffcompares.com/doc/g/grease-vs-oil.htm

https://www.quirkyscience.com/motor-oil-and-grease/

https://www.solverchem.com/articles/lubricating-oils-articles/what-is-sodium-soap-grease-composition/details

https://www.efficientplantmag.com/2009/07/grease-basics/

https://www.farmoyl.com/resources/common-grease-types-and-characteristics