Understanding Viscosity: What do those Numbers Mean?
When choosing engine oil or hydraulic fluid, one of the first things you'll notice is a set of numbers like 15W-40 or 10W-30 printed on the container. These numbers represent the viscosity grade, one of the most important factors in selecting the right lubricant for your equipment. But what do those numbers really mean? And how do they affect the performance of your machinery?
Let’s break down those numbers.
What is Viscosity?
Viscosity is a measure of how thick or thin an oil is, or more technically, how resistant it is to flow.
Thicker oil (higher viscosity) flows more slowly and provides a stronger lubricating film.
Thinner oil (lower viscosity) flows more easily, especially in cold conditions.
The key is finding a balance between cold-start flowability and high-temperature protection, especially for equipment operating in harsh Australian conditions. In recent years, a trend has emerged from major manufacturers to specify thinner oils to obtain fuel consumption savings.
So, what does 15W-40 even mean?
Take 15W-40 as an example. This is a multi-grade oil, and each part of the number has a purpose.
15W = Cold Weather Rating
The “W” stands for Winter. The number before it (15W) indicates the oil’s viscosity at low temperatures.
The lower the number, the better the oil flows in the cold.
For example, 5W-40 flows more easily in cold starts than 15W-40.
This matters particularly because machinery operating early in the morning or in colder regions. The quicker your machinery is lubricated, the less wear at startup!
40 = High-Temperature Rating
The second number (40) shows the oil’s viscosity at operating temperature (around 100°C).
The higher the number, the thicker the oil stays when hot.
Thicker oils are ideal for heavy-duty engines under high load or in hot climates.
So, a 15W-40 oil is thicker at operating temperature than a 10W-30 oil, providing better protection in extreme working environments.
Why Viscosity Matters for Your Equipment
Using the wrong viscosity oil can lead to:
· Cold-start wear
· Inadequate lubrication under load
· Reduced fuel efficiency
· Increased engine deposits
· Premature equipment failure
Whether you're maintaining a fleet of trucks, operating excavators on-site, or running agricultural gear, selecting the right viscosity is critical for engine life and efficiency.
Choosing the Right Viscosity for Your Operation
At the Green Goanna, we recommend choosing oil based on:
· Manufacturer specifications (always check your equipment’s manual)
· Operating temperature and climate
· Engine workload and hours
· Oil change intervals and service schedules
We stock a wide range of multi-grade oils from trusted brands like Shell, Castrol, Fuchs, and ENI, including popular viscosities like:
15W-40; a robust all-rounder for diesel engines.
10W-30; fuel-efficient and ideal for newer low-emission engines.
5W-30; great cold-start performance for light commercial vehicles.
Remember, viscosity isn’t just a number; it’s the key to keeping your engine running smoothly, efficiently, and safely. Choosing the right viscosity oil ensures longer service life, less downtime, and maximum protection. Need help choosing the right oil for your equipment?