What Does Viscosity Mean (and How Does it Affect Your Engine)?

Why does honey flow more slowly than water? Because of the difference in its viscosity.

Viscosity is the measure of a fluid’s resistance to flow. Water, for example, flows much faster than honey or maple syrup (if you prefer that in your morning coffee) when poured from a container.

That’s boil viscosity compared to honeyecause water has a lower viscosity. The lower the viscosity of a fluid, the faster it flows. Of course, the opposite is true. It’s often easier to think of viscosity in the following terms.

  • Thin and light describe fluids with low viscosity
  • Thick and heavy describe fluids with high viscosity

What causes differences in viscosity?
Molecules, honey (get it?)

I think best in visuals, so 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. If you have questions, contact AMSOIL Technical Services at [email protected] or 715-399-TECH.

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  1. There is no substitute for viscosity. For years that was the standard belief on lubrication (I’m 65). When I was young, 10w40 was the main choice although some still used straight weights like 20w20 and 30w. Over the years I’ve watched oil recommendations get lighter and lighter. At first I balked because I didn’t trust the lighter grades. Thought the car makers were just cooking up ideas to make them wear out faster so they could sell more. Over time it seemed that these lighter oils was doing the job so I finally gave in. I’m currently using 0w20 synthetic in my Tacoma. Now, I see that 0w16 is coming and there is talk of 0w8. Just how low can you go and still protect the engine?

  2. Is it Okay to Use AMSOIL Signature Series 0W-30 Full Synthetic Motor Oil,I have a 2013 Toyota Tacoma PreRunner w/ 4.0 V6 A.T. Tow Pkg SR5. Have 64,000 miles, I have been using Signature Series 5W-30 Motor Oil since I had 5,000 miles on from that day on I drained and refilled every 5-10,000 miles ALWAYS and with a K&N Oil Filter, the white one #1002! Please respond,I run all and Only AMSOIL since 5,000 miles, in my Brake Fluids,Pwr Steering Fluids,ATF Fluids,Rear Axle Fluids,All the Top Tier best as Severe Gear,Signature Series, Coolant Boost, Slip Lock Additives, I flush and fill every 25,000 miles all fluids and coolant,Brake Fluid,Tranny Fluids,Rear axle Fluids,Eng. every 5,000-10,000 w/K&N Oil Filters, Have a K&N Air Filter clean once a year ALWAYS.I am Old School,Did a tune up at 50,000 with all NGK Iriidium Top Teir Best Spark Plugs for my Truck, Runs better than new, Great MPG,Great Ride, Runs cool in HOTT Florida Summer Heat on Long roadtrips and in heavy traffic in Orlando and Tampa and more! Old School Diesel/Gas and heavy equipment Retired Mechanic for over 35 years !

    1. Hi Bob,
      The Signature Series 0W-30 will be fine to use in this Pre-Runner engine. For any applications that call for a SAE 5W-30 engine oil, the Signature Series 0W-30 will be able to be used.

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