Penetrating oil is very low viscosity oil. It can be used to free rusted mechanical parts (such as nuts and bolts) so that they can be removed, because it can penetrate into the narrow space between the threads of two parts. It can also be used as general purpose lubricant, or to clean or prevent corrosion. Use as a general-purpose lubricant is not advisable because it contains high percentage of volatile solvents and leaves only very small amount of lubricant.

WD-40 and CRC 5-56 are well-known brands of penetrating oil.

See also

Petroleum products | Lubricants | Oils | Kriechöl