Understanding Engine Oil Viscosity Grades: 15W-40, 10W-40 & 10W-30 Oils
For operators, fleet managers, and equipment owners, understanding engine oil viscosity grades such as 15W-40, 10W-40, and 10W-30 is essential for ensuring optimal engine performance and longevity as they provide critical information regarding the oil’s behaviour under varying temperature conditions. A link to our free easy-reference wall chart can be found towards the end of this article.
What Engine Oil Viscosity Grades Indicate
Engine oil viscosity grades are defined by the Society of Automotive Engineers (SAE). They describe the oil’s thickness, or resistance to flow, at both low and high temperatures.
Using 15W-40 as an example:
- 15W – The “W” denotes Winter, indicating the oil’s performance in cold conditions. The lower the number (such as 10W or 5W), the thinner the oil, so it flows more easily in cold weather, allowing for a faster start-up.
- 40 – The second number represents the oil’s viscosity at operating temperature. Higher numbers indicate the oil maintains greater thickness under heat, providing protection under load.
In essence, 15W-40 oil is slightly thicker during cold starts compared to 10W-40, but both grades offer robust protection once the engine reaches normal operating temperatures.
The Importance of Viscosity for UK Machinery
Engine oil functions as the primary protective medium for moving components. Selecting the correct viscosity ensures rapid circulation on start-up while maintaining adequate lubrication during operation.
Incorrect oil grades can result in slow engine starts, suboptimal lubrication, or increased wear. HTS Spares frequently encounters cases where machinery experiences start-up difficulties in cold weather due to inappropriate oil selection. Switching to a suitable 10W-30 multigrade oil, such as those available in the Kompo® range, often resolves these issues, improving operational reliability and engine longevity.
Common Engine Oil Grades and Applications
Below is a guide to commonly used engine oil viscosity grades in the UK and their typical applications:
- 15W-40: Heavy-duty diesel engines in construction and plant machinery. Reliable across typical UK temperature ranges.
- 10W-40: Versatile for mixed fleets or machinery operating in variable climates.
- 10W-30: Recommended for newer engines and colder environments, ensuring rapid oil circulation during start-up.
- 5W-30: Predominantly used in modern vehicles, providing efficient cold start performance and enhanced fuel economy.
HTS Spares’ Kompo® range is available across these grades, offering a trusted, high-quality option for plant, construction, and agricultural machinery.
Download our Free Wall Chart
At HTS Spares, we have extensive experience supplying lubricants and components for construction machinery throughout the UK and pride ourselves on simplifying things for our customers. To make it easier to identify the correct engine oil for each application, we have produced a free, easy-to-read Engine Oil Viscosity Wall Chart.
This printable reference guide summarises common oil grades, operating temperature ranges and flow rates - ideal for workshops, depots, or site offices.
👉 Download the Free Wall Chart Here and keep it close at hand for quick, confident decisions on engine oil selection.
HTS Spares - Your Reliable Engine Oil Supplier
Choosing the correct engine oil is critical for ensuring engine protection and performance. At HTS Spares, we emphasise that oil should be regarded as an essential component of maintenance strategy rather than simply a consumable.
Selecting the correct viscosity grade reduces wear, prevents costly downtime, and extends machinery service life. Using premium-grade products, such as Kompo® oils and greases, ensures consistent performance under the most demanding conditions.
Explore our full Kompo® range of engine oils and greases or contact our Kompo® technical advisor, Travis, for guidance on selecting the ideal grade for your equipment - 01432 373350.
