How To Read Oil Viscosity

You’ve probably been in an auto parts store and have been overwhelmed with all the brands, blends and viscosities that engine oil comes in. 5w-40, 10w-30, SAE 30, what does it all mean? This article will focus on how to read oil viscosity.

First off, what is viscosity?
Viscosity is the state of being thick, sticky, and semi-fluid in consistency. It’s the extent to which a fluid resists a tendency to flow. For example, maple syrup (high viscosity) is more viscous than water (low viscosity). This can be seen by pouring both out of a cup; the syrup is more viscous and will pour out more slowly than the water.

When talking about engine oil, viscosity refers to the resistance to flow and shear. Oil viscosity also determines how easily oil is pumped to the engine internals, how easily it will pass through the oil filter, and how quickly oil will drain back to the engine oil pan.

Where did these numbers come from?
The Society of Automotive Engineers (SAE) established this numerical system to measure engine oil by its viscosity characteristics.

How do you read viscosity?
The first half typically includes a number and the letter “W”. This “W” stands for “Winter”. The number in front of the “W” refers to the oil’s cold weather viscosity. The lower the number, the less viscous (runnier) that oil is at low temperatures. A 5W- engine oil will flow better at lower temperatures than a 15W- oil.
The second, higher number refers to hot weather viscosity; or how viscous the engine oil is at hot temperatures (operating temperature). The higher the number, the thicker the engine oil will be in high temperatures.

Why do some oils only have one weight listed?
On a multigrade oil you will see two numbers specifying viscosity, I.E.: 10w-30
Monograde or straight weight oil contains only one specification I.E.: SAE 30

Please note: If your owner’s manual calls for a multigrade oil, monograde oils are never recommended. Most monograde oils are used by small seasonal engines or older vehicles before multigrades were produced.

Other common questions:

What is TBN?
TBN stands for Total Base Number. TBN is a measure of how much active detergent and dispersive additive is left in your engine’s oil, this number is determined by an oil analysis.

What is API?
API stands for the American Petroleum Institute, which is the leading US trade association for the oil and natural gas industries. API helps to set standards for production, refinement, and distribution of petroleum products.

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