Penetrating oil
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.
WD-40 is a well-known American brand of penetrating oil, though it is so light that use as a general-purpose lubricant is not advisable.