Viscosity is the Most Important part of a Lubricant

Amsoil Technical Director Dan Peterson


Viscosity is one of the most important yet misunderstood physical properties of a lubricant.

Most consumers don’t understand the true nature of multi-viscosity lubricants.

Without a thorough understanding of viscosity tests and classification systems it is difficult to explain the specific reasons different applications and conditions call for different lubricants. Viscosity is defined as a fluid’s resistance to flow, and some fluids have a higher viscosity than others. For example, honey is much more viscous (flows more slowly) than water.

Most vehicle owners understand their application requires a specific viscosity grade and type of oil, and that those details are usually outlined by the original equipment manufacturer (OEM) in the owner’s manual. But what do these different viscosity recommendations really mean? What is the difference between 5W-30 and 10W-30 engine oil?

First, there is no difference between a 5W-30 oil and a 10W-30 oil when the engine has warmed up – the second number of a multi-viscosity oil represents the oil’s thickness at operating temperature. So, a 5W-30 oil and a 10W-30 oil both have the same viscosity (resistance to flow) when the engine is running. It is the number before the dash that differentiates them. This number, known as the winter viscosity grade, tells us what the oil’s thickness will be when the engine starts, when it is cold. The smaller the “W” number, the easier an engine starts in cold temperatures. So, a SAE 5W oil is less viscous and will flow more easily than a SAE 10W oil in cold temperatures at startup and during warm-up. Most engine wear occurs at this critical time because of insufficient oil flow.

There are a number of different viscosity grading and classification systems used for lubricants. One of the most common systems is the SAE J300 viscosity grading system for engine oils. This table outlines specific requirements for viscosity at different temperatures and shear conditions. So to determine the difference between 10W-30 and 5W-30 engine oil, we first need to understand two basic viscosity measurements and the shear test used to measure the viscosity stability of engine oils. The most common measurement of viscosity is Kinematic viscosity, which is measured at 100°C (212°F) – close to the maximum operating temperature for most passenger car and truck applications. This test measures how fast an engine oil is pulled by gravity through a glass tube heated to 100°C. The time it takes for the oil to move through the tube is converted to a viscosity measurement called centistokes (cSt). The SAE established categories of oil based on this measurement. To obtain the 30 in 5W- 30, 10W-30 or any other SAE 30 weight designation, the Kinematic viscosity at 100°C must be at least 9.3 cSt and no more than 12.5 cSt.

In addition to Kinematic viscosity requirements, each viscosity classification has high-temperature/high-shear (HT/ HS) requirements. The HT/HS test simulates an engine at operating temperature and under load to measure how well oils maintain thickness over time. It is determined at 150°C (302°F), and a SAE 30-grade oil has a minimum HT/HS viscosity requirement of 2.9 cSt.

So, how is the winter viscosity grade determined? This cold-temperature designation uses another viscosity measurement called Brookfield viscosity. Brookfield viscosity is measured in centipoise (cP) at designated temperatures for each W rating. Using the honey example, Brookfield viscosity simulates stirring a bowl of honey and measuring the amount of resistance to stirring in the bowl. Thicker honey takes more energy to stir, and honey becomes thicker the colder it gets. For a 30-weight oil to also qualify for a 10W rating, the Brookfield cranking viscosity (ASTM D-5293) cannot exceed 7,000 cP measured at -25°C (-13°F), and the Brookfield pumping viscosity (ASTM D-4684) must be less than 60,000 cP at -30°C (-22°F). Alternatively, in order to qualify for a 5W rating the Brookfield cranking viscosity cannot exceed 6,600 cP measured at -30°C, and the Brookfield pumping viscosity must be less than 60,000 cP at -35°C (-31°F).

Several other viscosity classification systems are used to designate different grades of lubricants. Another common system is the SAE J306 system, which defines automotive gear, axle and manual transmission lubricant viscosities and incorporates both the SAE rating and winter viscosity requirements similar to the SAE J300 engine oils system.

Industrial fluids generally use a system designated D 2422-97, which separates different industrial fluids into different ISO viscosity categories. These categories are commonly referred to as ISO viscosity grades and run from ISO VG 2 through ISO VG 3200 (ISO stands for International Organization for Standardization, VG stands for viscosity grade). A common hydraulic fluid viscosity grade is ISO VG 46, where the mid-point Kinematic viscosity at 40°C (104°F) is 46 cSt, and the range to qualify for this viscosity category is 41.4 to 50.6 cSt.

On the surface, viscosity may seem like an easy concept to understand, but most consumers don’t really know where to start. Different rating systems are used for passenger car motor oils, gear lubes and industrial lubricants. Although it is not necessary for Dealers to know all of the technicalities, a basic understanding is important in order to make correct viscosity recommendations for customers.

Author: johnzena

Amsoil dealer in Las Vegas NV. Plus I work as a dice dealer at the Bellagio.

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