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The Importance of Viscosity in a Lubricant

Viscosity, the measure of the internal friction of a liquid (resistance to flow) is one of the most important characteristics to consider when determining which fluid is most suitable for an application.

How the the viscosity of a fluid reacts to changes in temperature, pressure or speed determines how well a fluid can perform the basic functions of a lubricant.

Too High vs. Too Low

A viscosity that is too low for a specific application leads to

Increased metal-to-metal contact

Increased friction and wear

Increased oil consumption

Leaking seals

Viscosity that is too high for a specific application leads to

Increased fluid friction

Increased operating temperatures

Poor cold temperature fluidity

Reduced energy efficiency

The proper viscosity for a lubricant to be most effective must be able to keep moving parts separated under normal operating temperatures, pressure and speed.

Specific Factors Effecting Viscosity

Temperature

Lubricant base oils thicken as they cool. As the temperature continues to drop the base oil will eventually become solid. This thickening increases the lubricants load carrying ability, but its ability to flow well enough to circulate to the areas where it is needed is reduced.

On the other hand, when heated a lubricant’s base oil will thin out which decreases its ability to carry a load and prevent metal-to-metal contact.

Pressure

Individual applications require specific viscosities to ensure that forces applied to the oil don’t result in excessive component wear.

If a lubricant with too low of a viscosity operates in an extreme pressure environment it will loose its film strength (called mechanical shear) leading to metal to metal contact.

A lubricant with too high a viscosity will not flow well through small oil passages resulting in an inadequate amount of the lubricant reaching the components needing protection.

Speed

Whatever the application, the lubricant’s optimal viscosity must also consider the speed at which the application operates.

As speeds increase, lower viscosity oil may be necessary to insure optimal operational efficiency.

If the viscosity of a lubricant is too high in a high speed component its film thickness may increase to the point where excessive heat is generated and the life of the oil will be shortened.

Viscosity Control in Lubricant Development

AMSOIL Synthetic Motor Oils are designed to deliver optimum viscosity for their intended applications. Advanced base-oil formulations and additive packages provide robust protection against viscosity breakdown for reliable protection and performance.