A Guide To Car Engine Oil Viscosity Grades

0W, 10W, 20W, 5W-40, 5W-20, 0W-30, 15W-40 — what do all these characters mean? You’ve likely seen one of these or a variation of them on your preferred car engine oil label.

If you’re like most motorists, you probably don’t know what these numbers mean. That’s perfectly fine. As a car owner, you only have to abide by one simple rule when it comes to engine oils. Just follow the viscosity grade indicated on your car owner manual. Do that, and you’ll be fine.

However, if you want to understand why you must strictly adhere to your car owner manual’s viscosity grade prescription, then you need a crash course on engine oil viscosity grade and why it matters.

What Is Viscosity Grade?

First, a brief discussion of car engine oils is warranted. Engine oils are lubricants. They coat engine parts to reduce friction, which could lead to overheating and the premature wearing out of these parts, which will damage the engine. Engine oils also help keep engines clean through additives that dissolve deposits and sludge, which could eventually lead to poor engine performance or engine malfunctions.

Engine oils, to be effective in lubricating and protecting engines, must have a certain level of viscosity. Viscosity is thickness. Hence, a car engine oil’s viscosity grade will tell you how thick that engine oil is. Put another way, an engine oil’s viscosity refers to how resistant to flow that engine oil is.

The thicker an engine oil, the more resistant to flow it is. Hence, the greater its lubricating powers and protective abilities.

Is higher viscosity better? Generally it is, but not always. In higher temperatures, it is, but in colder climes, it is not.

When it’s freezing, thicker engine oils might not flow quickly enough to protect critical engine parts during a cold engine start. Thus, in such conditions, you actually want thinner rather than thicker oils. It is the opposite in hot climates. The heat causes engine oils to thin out, so a thicker engine oil is better then.

In other words, the temperature is always a factor when choosing the appropriate viscosity grade — but more on this later.

Now that you know viscosity is resistance to flow and the more viscous an engine oil is, the better it is generally for the engine, how do you relate this information to the viscosity grade labels on engine oils?

The viscosity grade label on engine oils will tell you two things: the engine oil’s viscosity grade and whether the engine oil is a monograde or a multigrade oil.

Monograde Or Multigrade?

Engine oil can be monograde or multigrade.

Monograde Oils

Monograde oils have only one number on the label. They may or may have a “W” designator. For instance, if an engine oil is labelled 40, 30, 20, 0W, 5W, 10W, 15W, 20W (with or without SAE, which is an acronym for Society of Automotive Engineers), that is a monograde oil.

Monograde oils are usually made for older engines that operate in a limited temperature range (e.g., in the cold or in the heat). The number refers to the engine oil’s viscosity grade. The “W” designation refers to winter. Therefore, if your engine requires a monograde oil, choose a “W” variant in the winter. In the summer or warmer months, choose a monograde oil without the “W.”

Multigrade Oils

Multigrade oils have two sets of numbers before and after the letter “W”. The letter “W” still refers to winter. The first number, found on the left of the “W,” pertains to the engine oil’s viscosity grade in low temperatures. The second number, located after the “W” (typically a dash, too), is the engine oil’s viscosity in higher temperatures.

Viscosity Grade

As you might have guessed, the number on the viscosity grade label refers to the actual viscosity rating of the engine oil. However, for rating standardisation purposes, an engine oil’s viscosity is measured at specific reference temperatures.

The Number Before the “W”

The viscosity rating that comes before the “W” in multigrade oils and with “the letter “W” in monograde oils refers to the engine oil’s viscosity grade at 0°F. In other words, it tells you how thick the oil is at low temperatures or when it’s freezing outside.

The Number After the “W”

The number that comes after the “W” in multigrade oils or the number that stands alone (without the “W”) in monograde oils is the engine oil’s viscosity at 212°F. This is the engine oil’s viscosity at higher temperatures.

Higher Or Lower Viscosity Grade?

Low temperatures can lead to the excessive thickening of engine oils, and excessively thick engine oils will make it more difficult to start the engine at low temperatures. Thus, in the winter, or if you live in a place where below freezing is the standard temperature, you need a lower “W” viscosity rating. A lower “W” viscosity grade (e.g., 0W and 5W) means the oil will flow easier even in a cold start.

High temperatures, meanwhile, thin engine oils. Excessively thin engine oil will offer subpar engine lubrication or protection. Therefore, if you’re operating your vehicle where it’s hot or if your engine gets excessively hot (for instance, your hobby is circuit racing), you want a higher viscosity grade (e.g., 30, 40, 50).

Then again, just follow what your car owner manual says, and it’s all good.

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