Penetrating oil, also known as penetrating fluid, 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 a general-purpose lubricant, a cleaner, or a corrosion stopper.
Using penetrating fluids as general-purpose lubricants is not advisable, because such oils are relatively volatile. As a result, much of the penetrating oil will evaporate in a short amount of time, leaving little residual lubricant.