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

Motor oil viscosity is the measure of its resistance to flow. Low-viscosity oil (e.g. 0W-20) flows faster than high-viscosity oil (e.g. 20W-50). In this post, we explain everything you need to know about oil viscosity.

To illustrate, think of water and honey. When poured from a container, water flows much faster than honey.

Kinematic Viscosity-Oil vs Honey

That’s because, when external forces like gravity act on a fluid, the molecules within the fluid move against each other, resulting in molecular friction that resists flow.

Viscosity is a measure of that internal friction, or its resistance to flow.

It’s helpful to think of it in these terms:

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

Motor oil viscosity chart

Motor oil viscosity is often reported using the Society of Automotive Engineers (SAE) J-300 chart. The chart shows the minimum and maximum allowable thresholds a motor oil must meet to be rated for the specified viscosity.

Motor oil viscosity chart

The oil’s winter, or “W”, rating, is determined based on its cold-crank performance, which simulates an engine turning over at progressively colder temperatures. The oil’s ability to flow at progressively colder temperatures is also measured. The lower the “W” rating (e.g. 0W), the faster the oil flows when cold and the easier your engine turns over to start.

The second number (e.g. the “20” in 5W-20) is determined based on the oil’s viscosity once your engine reaches operating temperature, or 100ºC (212ºF).

What does oil viscosity mean to engine protection?

So, what does this all mean to protecting your engine?

Put simply, viscosity is the most important property of a lubricant. How it 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 and wear
  • Increased oil consumption, which leads to harmful deposits and frequent top-offs
  • Leaking seals

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

  • Increased fluid friction, reducing fuel economy
  • Increased operating temperatures, hastening oil breakdown
  • Poor cold-temperature starting

Oil thickens when cold…

When the temperature drops in winter, motor oil thickens, flows more slowly and requires 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.

Check out the video to see the difference in cold-flow between AMSOIL synthetic motor oil and a conventional motor oil.

If your oil flows more slowly, engine components may be vulnerable to wear until the oil warms enough to flow throughout the engine.

As the video shows, synthetics are superior to conventional oil in this regard. That’s why it’s best to use a lower viscosity oil in winter if your vehicle manufacturer allows it.

…and thins when hot

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 thin. 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, and the better it maintains separation between moving parts.

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 oil 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.

Originally published Sept. 2, 2016.


  1. Thanks John. I have a question, I’ve modified the engine of my scooter; replaced original piston to a high comp piston then, ported the head. Based on your article, “But keep in mind that your vehicle’s viscosity requirements may change if you’ve modified your engine.” – does it mean I have to change the viscosity requirement of my motorcycle?

    1. Hi Kevin,

      To answer that question, we’ll need more information. Contact AMSOIL Technical Services and they’ll be glad to help ([email protected]; 715-399-TECH).



  2. For what you charge for a quart of oil I do not feel it is worth it compared to a good quality synthetic for half the price.

  3. Hey John,

    This has broadened my knowledge in this section.

    Thanks for sharing the viscosity chart it’s clear enough for anyone to understand.

  4. 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?

  5. 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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