Oil viscosity is the most important property of a lubricant.
Understanding viscosity promotes the ability to reduce wear, improve fuel economy, and make more horsepower.

For starters, in oil nomenclature, “W” does not stand for “Weight”.
It stands for “Winter” and that is the key to understanding viscosity grades.
A 10W-30 is a multi-grade (two viscountcies) motor oil, and as the name implies, it meets more than one grade. Forty years ago, there were winter grades for cold weather and summer grades for warmer weather.
A typical winter grade was 10W. A typical summer grade was 30.
These oils were straight grade oils. A 10W flows well in cold weather, to protect the engine at start up, but it is too thin for use in the summer.
A 30 grade oil, thick enough to protect in the heat, was recommended for summer use.

Then, multi-grade oils were formulated. A 10W-30 had the winter cold start flow properties of a 10W and the summer, high-temperature thickness of a 30 grade.
Multi-grade oils could stay as close to the optimum viscosity over a range of temperatures, not too thick when it is cold and not too thin when it is hot.

The difference between a 0W-30 and a 10W-30 is indicated by how well each flows at lower temperatures. The viscosity of hot oil is measured using different test parameters than when the oil is cold, so the numbers after the “W” don’t relate to the numbers in front of the “W”.
The difference between 10W-30 and a 10W-40 is the high temperature viscosity. Obviously, a 10W-40 is thicker than a 10W-30 at high temperature.

Armed with knowledge of viscosity grades, how can we put it to good use?
Remember that using oil with a viscosity that is too high can result in excessive oil temperature and increased drag.
Using an oil with a low viscosity can lead to excessive metal to metal contact between moving parts.
Using the correct viscosity oil eases starting, reduces friction, and slows wear.

For even more effective start-up protection, use a synthetic 10W-40 instead of a conventional 20W-50.
The synthetic 10W-40 flows easily and still maintains enough viscosity to protect piston skirts and bearings when it gets hot.
The improved temperature stability of synthetics makes them a better choice for race engines and serious high-performance engines.

Even with a synthetic, however, viscosity changes with temperature. Selecting the correct viscosity for an application requires knowing the operating temperature of the oil. Engines that run high operating oil temperatures require higher viscosity oil.

The difference between a 0W-30 and a 10W-30 is indicated by how well each flows at lower temperatures.
The viscosity of hot oil is measured using different test parameters than when the oil is cold, so the numbers after the “W” don’t relate to the numbers in front of the “W”.
The difference between 10W-30 and a 10W-40 is the high temperature viscosity.
Obviously, a 10W-40 is thicker than a 10W-30 at high temperature.

Armed with knowledge of viscosity grades, how can we put it to good use? Remember that using oil with a viscosity that is too high can result in excessive oil temperature and increased drag. Using an oil with a low viscosity can lead to excessive metal to metal contact between moving parts.
Using the correct viscosity oil eases starting, reduces friction and slows wear.

Armed with knowledge of viscosity grades, you can now select the right one for your application.
In return, you will prevent wear, improve fuel economy, and make more horsepower

Considering there are multiple standards (that use different scales) for designating viscosity, a comparative viscosity chart can help determine if two or more lubricants have similar viscosities.
But how do you read a gear oil viscosity chart?

Just read it horizontally. For example, an SAE 60 motor oil has a similar viscosity to an SAE 90 gear oil, an AGMA 6 gear lubricant and an ISO 320 hydraulic fluid/gear lubricant. The corresponding kinematic viscosity and Saybolt viscosity are also referenced on the chart.

Kinematic Viscosity

Commonly seen on a lubricant’s data sheet, kinematic viscosity describes a fluid’s visible tendency to flow.
Think of this as the time it takes to watch a fluid pour out of a container.
This tendency to flow is expressed in units suggesting the volume of flow over time, called centistokes (cSt). Kinematic viscosity is usually tested at both 40°C and 100°C.

Saybolt Viscosity

Although centistokes are the most common unit of measurement when determining kinematic viscosity, results may also be reported in Saybolt Universal Seconds (SUS). Viscosity reported in SUS is becoming increasingly rare, but you may still come across it when reading lubricant product information.
Saybolt viscosity is usually tested at both 100°F and 210°F.

Comparative Gear Oil Viscosity Chart