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Lubrication can be divided into two categories, hydrodynamic and boundary. Hydrodynamic is the liquid barrier that comes from a layer of oil between two moving parts. Sooner or later, that hydrodynamic layer fails. Direct metal to metal contact is rare and is usually momentary, but that wear is cumulative. That's where the boundary lubrication comes in. Boundary is created by a higher viscosity oil that stays in place longer (sticking to parts) and creating a protective surface for the parts to slide. The big problem is, this boundary lubrication depends on the various Extreme Pressure (EP) additives in the oil. These additives create a layer, sometimes only molecules thick, between the metal parts. The problem with oil is, the additives is the weak link. Theoretically, oil itself does not break down; it is the additives that become depleted as they combat contaminants and heat. The majority of the contaminants are the acids that are byproducts of the internal combustion engine. As the additives negate the additives, they deplete themselves and it is the depleted additives that turn your oil black. With that said, there are more of this combustion acid at lower operating temperatures and at non-constant engine speeds. At higher operating temperatures and at constant engine speeds, the engine is more efficient and less acids are produced. Keep in mind also that too high of temperatures will cause additives to break down too.
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