What Is Penetrating Oil?

If you are an auto hobbyist, young machinist, do-it-yourselfer or avid tinkerer, you might have heard the term "penetrating oil." Penetrating oil is an umbrella term that refers to a class of lubricants with a thin, water-like consistency, such as WD-40, Kroil, PB Blaster and Zep. These low-viscosity lubricants are different than thick-film or “fluid” lubricants, and they play a critical role in engineering.

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A penetrating oil is a petroleum-based, low-viscosity lubricant. Its low viscosity lets it seep into bolt treads and other tight spaces, allowing for looser, easier movement.


What sets penetrating oils apart from other classes of lubricants is their low viscosity. Viscosity refers to a liquid's or semi-solid's resistance to flowing when a force such as gravity, pressure or a siphoning effect is applied to it. Part of a class of petroleum-based lubricants known as thin-film lubricants, penetrating oils' low viscosity is on par with water. Because of this, penetrating oil comes in liquid and aerosol forms. Finally, as a petroleum product, penetrating oil is highly water-repellent.

Standard Uses

Commonly used in the automotive, energy and manufacturing industries, penetrating oils have three general applications: lubricating rapidly moving or sensitive machine parts; loosening treads on bolts, axles and other fasteners; and as a water repellent/anti-corrosive coating. The low viscosity allows the oils to lubricate without adding resistance between parts. Also, with aerosol spray, it's much easier to apply and reapply the penetrating oil to hard-to-reach places. When applied to stuck treads, the low-viscous oil will seep deep into the narrow places (hence the term "penetrating") to dissolve encrusted residue and free the pieces. If a tightly fit moving part is making noise, penetrating oil applied to the outside will seep into the microscopic space between the metal as a lubricating layer. Given their tendency to expand across surfaces in a uniform coat (caused by a phenomenon known as weak surface tension) as well as their natural ability to repel water molecules, penetrating oils are effective at protecting iron and aluminum from rust. Machinists and engineers are well-aware of this added benefit, and frequently use it as a protectant for commercial and industrial products.

Non-Standard (But Still Pretty Cool) Uses

With the exception of the 10 percent to 15 percent in inert materials and up to 20 percent in liquified carbon dioxide, penetrating oils are composed entirely of hydrocarbons. Hexane, which is a liquid under normal conditions, happens to be an excellent nonpolar solvent. Water, a polar solvent, is good at dissolving and breaking up certain substances such as salts, sugars and metal ions, but ineffective at dissolving oil, fats, plastics or other nonpolar solvents. Therefore, penetrating oils can be used to remove chewing gum, stickers, permanent marker, dripped varnish, acryllic paint, spray paint and other nuisances that can't be easily broken up by water.

Top Brands

WD-40, literally the inventor's 40th formula for a water-displacement solution, is the oldest and most familiar brand of penetrating oil. However, newer brands, most notably Kroil, PB Blaster and Zep, are strongly preferred by mechanics and engineers for their lower viscosity, resistance to evaporation and superior solvent strength.


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