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).
To illustrate, think of water and honey. When poured from a container, water flows much faster than honey.
That’s because, when external forces act on a fluid (such as gravity), 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.
Its 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.
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 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 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.