Gold can be used as lubricant in a number of engineering applications based on its resistance to cold welding

Control of friction is a key engineering requirement in many applications. Under low pressures films on metals which are lost as a result of friction or other effects are not replaced, with the result that metal surfaces in contact with each other can easily cold weld to one another. In space, this difficulty is increased since in some external situations non-metallic lubricants may suffer from radiation damage, chemical breakdown or evaporation.

As a result of its low shear strength, cold welding is not the serious problem with gold that it is with other metals in such situations, and it has therefore been applied as a solid film lubricant in many space situations involving moving parts, both alone as a surface coating on the bearing surfaces, and in combination with low volatility synthetic lubricants.

The lubricating properties of gold are also now being exploited in a niche commercial application in China. The NanoGold OilTM is an additive for petrol or diesel engines that is claimed to offer fuel savings and power improvements, as well as reduced engine noise. Traditionally graphite can be used as an ingredient of cylinder coating additive, along with polytetrafluoroethylene (i.e. Teflon). The manufacturers claim that in fact gold is the best material for a cylinder coating additive but is rarely used due to its high price and limitations of milling technology. NanoGold OilTM is based on a liquid additive comprised of synthetic oil and gold nanoparticles that are believed coat a layer of gold on the engine’s cylinder and piston surface, as well as on lower parts of engines such as shaft and cam shaft, thereby filling and sealing micropores and pits that may exist. For more information see http://www.nano95.com.tw/eng.htm
A more detailed explanation of the use of gold films as lubricants can be found in this paper published in Gold Bulletin, see http://www.goldbulletin.org/downloads/Antler_2_21.pdf