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Lubricant Viscosity Explained

 

Every type of fluid possesses differing amounts of resistances against deformation. The measure of that resistance is called viscosity. Viscosity expresses the fluid’s resistance against either tensional stress, or shear stress.

A lubricant’s viscosity and how it changes under different temperatures and operating conditions is one of the most important properties that determines lubricant performance and protection.

Viscosity can be viewed in two ways:

Kinematic viscosity (also called "momentum diffusivity") 

is defined by the lubricant’s resistance to flow and shear due to gravity. To illustrate, imagine pouring two containers, one filled with water and the other with honey. Each fluid’s Kinematic viscosity governs the rate at which it flows. Since the Kinematic viscosity of water is lower, it flows faster. For a liquid, the kinematic viscosity decreases with higher temperature. Kinematic viscosity, measured using ASTM D445 methodology, determines an SAE oil’s high-temperature viscosity grade (the “30” in 5W-30).

Dynamic viscosity (also referred to as absolute viscosity)

measured by the Cold Crank Simulator (CCS) test (ASTM D5293), is defined as the lubricant’s resistance to flow as indicated by its measured resistance, best thought of as the amount of energy required to move an object, such as a metal rod, through the fluid. It takes less energy to stir water compared to honey because the Dynamic viscosity of water is lower. Dynamic viscosity determines an oil’s low-temperature grade (the “5W” in 5W-30).

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What causes differences in viscosity?

Let’s think of molecules as a group of people in a room. In one group everyone is holding hands very lightly. It’s flu season and they don’t want to get sick. On the other side of the room, everyone is holding hands fairly tightly. It’s a close-knit group, that one.

If you weave through both groups and break through the chains of people, you have an easier time weaving through the first group than the second. This is essentially how molecules are bonded in liquids. The looser the molecular bonds, the weaker the inter-molecular forces. This equates to a lower viscosity. The tighter the molecular bonds, the stronger the inter-molecular forces. This equates to a higher viscosity.

How does viscosity affect engine protection?

So what does this all mean to protecting your engine? Put simply, viscosity is the most important property of a lubricant. How lubricant viscosity reacts to changes in temperature, pressure or speed determines how well the oil protects your vehicle. Lubricants with too low of a viscosity for your engine may cause:

  • Increased metal-to-metal contact
  • Increased friction and wear
  • Increased oil consumption
  • Leaking seals

Lubricants with too high of a viscosity could also hurt your engine by causing:

  • Increased fluid friction
  • Increased operating temperatures
  • Poor cold-temperature starting
  • Reduced energy efficiency

How temperature affects viscosity

When cold, lubricants thicken and flow more slowly and require more energy to circulate. That’s why it may be tougher to start your car on a frigid winter morning – the crankshaft has to churn through cold, thick oil before it spins fast enough for the engine to start. Since the oil flows more slowly, engine components may be vulnerable to wear until the oil warms enough to flow throughout the engine.

The opposite happens when the temperature soars. Say you’re towing a camper down the interstate at the height of summer. The intense heat your engine generates causes the oil to become thinner. If it becomes too thin, it can fail to adequately separate metal components during operation, inviting wear.

The greater a lubricant’s viscosity, the greater pressure or load it can withstand, allowing separation between moving parts to be maintained. But there are limits to this relationship. If the viscosity is too high, it won’t flow as readily and your engine will work harder and burn more fuel.

Different vehicles require different viscosities

The key is to use a lubricant with the correct viscosity for the application. Not only that, but you want to use a lubricant that resists thickening when cold, yet maintains its ability to protect against wear when hot. Synthetic lubricants, such as AMSOIL synthetic lubricants offer better cold-flow when the temperature drops and improved protection once your engine has reached operating temperature.

Vehicle manufacturers specify in the owner’s manual which viscosity of motor oil you should use. You can always use the AMSOIL Product Guide to find that information, too. But keep in mind that your vehicle’s viscosity requirements may change if you’ve modified your engine.

 


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