Molybdenum disulfide, known as Moly or MoS2, has long been used as a friction reduction additive in personal and commercial vehicle engines and drivetrains. Stock factory constant velocity grease has always had MoS2 as an ingredient, and your CV joints would just not survive without it. The metal to metal stresses are just too high for regular grease without Moly. Many factory motors come with pistons that have a Moly coating on the skirts. Moly coating is a bit different than a moly solution for friction reduction. In high concentrations such as a coating, moly has a property called EP or Extreme Pressure lubricant. This can protect sliding surfaces during the moments when little or no oil is present to otherwise lubricate them. Many high end and high performance engine parts are available that come with a moly coating, generally .001″ to .003″ thick.
Moly has been used in plastics, rifles, and even ski wax. Moly also has uses in electronics such as photo detectors, memristors and field-effect transistors. A short summation is available in this Wikipedia Article on MoS2. Complimentary information is found at Engineers Edge. A very complete treatise can be found at the website for the International Molybdenum Association (IMOA).
Moly is not in the class of additives called durifiers. Last century, it was common to see advertisements for additives that formed a bond with surface metal, creating a durified layer. One firm claimed that after treatment they could drain all the oil and still drive from New York to Florida. Another filled the crankcase with water after a treatment, and “successfully” ran the motor for some time. Over time, it became obvious to some that these additives could eventually shear off in chunks, evidenced by extreme gauling of engine bearings. In some cases, the surface got harder than it should, and the top layer just came off. Most of these durifying additives are long gone, but they kind of lent the snake oil odor to other quality products.
Poorly understood is that inside a running engine, peak oil pressures can run between 5,000 and 40,000 psi. When the oil film between a rod bearing and a crankshaft gets squeezed by the force from a piston during its cycles, the oil film changes thickness and has a peak pressure wave that runs just in front of the force vector. This peak pressure wave nibbles at the bearings. When surfaces are perfect, very little wear occurs. Any imperfection may shed metal molecules. Reducing the friction and filling the imperfections by using a Moly additive can stop this process. And reducing friction will also increase power output and mileage. It is contestable whether any customer could accurately assess the improvements in power or mileage, as the actual increases vary and are very small, typically between 2% to 5%. Other effects of friction reduction are decreases in oil temperature by 20 to 30 degrees, and reduction in thermal loading of both radiators and oil coolers.
The Moly molecule (MoS2) has the molybdenum atoms sandwiched between triangular links to upper and lower tiers of sulfur atoms. The lubrication layer created depends on the sulfur atom of the outer tiers of the MoS2 molecules bonding to the metals with a firm grip. Then the outer sulfur tiers slip past each other with very little resistance, meaning low static and dynamic friction. A high magnification image of a crankshaft surface somewhat resembles a silhouette of the Himalayas. After Moly treatment, it looks more like the Sierra foothills. Completely filling the valleys is not necessary, as it is the tips of the foothills that actually try to contact. The excesses in the valleys however can move to refresh the particles that are slowly worn off the tips. The relative lifespan of this Moly layer changes with many variables. Cam to lifter film is very different than crank to bearing, as one is two metals scraping constantly, and the other has a pressurized oil film, and the two should never touch.
Why not use large amounts of Moly additive to protect the engine? There are a few caveats. Most importantly, Moly is a case where more is definitely not better. There is a range of concentration within which Moly is most effective, between 300 and 400 ppm (parts per million). In very high concentrations and with cold weather use that can bring condensation, Moly can contribute to the formation of slight amounts of sulfuric acid (H2SO4). The average particle size of moly in quality additives is reputed to be .05 microns, and the finest oil filters filter to 5 microns. But that is average particle size, some of it will be larger. Using too much additive or too often could build up enough larger particles to affect oil filtration or mobility. To generalize, one should add fresh Moly additive every 2 to 3 oil changes.
Better not to experiment with DIY Moly additives. Some have particulate sizes big enough to clog oil filters. Some mostly drop to the bottom of the oil pan. Any additive that says you need to shake it up before installing is likely not to stay in solution as it should. The best additives have these problems dialed out, and give excellent benefits with no repercussions. Liqui Moly is a quality German brand, made only in Germany, no foreign factories. The Liqui Moly product is called MoS2 Anti-Friction Engine Treatment. Schaeffer Oil sells Moly E.P. Oil Treatment (#132) which contains Micron Moly®. Schaeffer Oil is an American brand, and their products have been made in the USA since 1839. Both these additives have far more than just the Moly, and are geared towards proper Moly distribution as well as a good overall engine oil additive package.
MoS2 Transmission and Differential Gear Oil Additive
Friction reduction is beneficial to transmissions and differentials. Longer wear life and better shifting result from using a tube of Lubro Moly in the gear oil every 40,000 miles. Once plated with Moly additive, molybdenum disulfide forms an indestructible long-lasting film that is capable of withstanding pressures up to 500,000 psi. In theory, like in an engine, the gear surfaces are never supposed to actually touch, as the oil layer gives them the tiny bit of separation that keeps wear from happening. In practice, during the first few revolutions after sitting idle, during hard shock loading and sometimes during very high temperature usage, the surfaces will touch and wear. Even in those instances, the micro layer left by the moly plating will dramatically reduce or prevent the occurrence of wear and gauling.
One might think that the normal friction that makes a synchromesh work right might be affected. In fact, the best synchromeshes come coated with moly to improve the life and function. And it is common practice with racing transmission parts to either come with a moly coat or be sent out for professional coatings to be applied.
Liqui Moly Diesel and Gasoline Additives
Liqui Moly has a full range of products to clean and maintain engines, both diesel and gas. One their products is one of the few that actually can clean some pre-existing carbon deposits from gas engines. It is worth mentioning that gasoline engine customers who use high quality fuel that already has a cleaning and stabilizing additive package really only need to use any other additive once every year or two. Those who buy cheap gas will pay the difference at least ten fold once the repair bills to remove the carbon, fuel vapor varnish, and PCV grit start kicking in. We offer gasoline fuel additives, but only find them necessary for cars that have run on cheap gas.
Diesel engine customers should be adding an additive to every tank of fuel. Since the removal of almost all the sulfur from modern diesel, the lubricity is just not good enough to prevent long term fuel system wear. The soot reduction properties of additives also prevent clogging of catalytic converters, exhaust gas recirculation (EGR) filters, and diesel particulate filters (DPF). At Karmakanix, we offer Stanadyne and Liqui Moly additives for diesels. And we like to tell customers that they can purchase Redline brand diesel additives at any parts store, and thus keep a supply on hand no matter where they live or travel. Also of note is a Liqui Moly formulation for modern Common Rail (CR) diesel engines clearly designed to prolong the life of the Diesel Particulate Filter or DPF.