The importance of choosing the correct oil viscosity for your vehicle

Graphic of 4 test tubes with oil and ball bearings at different levels - showing different oil viscosities.

Did you know the choice of oil viscosity can have a significant impact on your vehicle’s overall performance, longevity, and efficiency?

We spoke to our Technical experts to explore what viscosity is, how it is measured and making the best decision for your vehicle.

Read below to find out more…


Understanding Viscosity

Viscosity is the measure of an oil’s thickness and how easily it flows at different temperatures. Engine oils naturally get thinner as they are heated and thicken as they cool. Thin, low viscosity oils tend to provide better engine protection in cold temperatures. Thick, high viscosity oils are typically better at maintaining film strength to reduce friction and mitigate wear, protecting engines at high temperatures.

Measuring Viscosity

Viscosity is measured using a standardised system known as the Society of Automotive Engineers (SAE) viscosity rating. Oils and fluids are classified into specified grades, the two main types being monograde and multigrade.


Monograde oils are often used in vintage and classic cars. Older vehicles possess features not found in modern cars including, hotter engines, and lower RPMs, all factors which make their lubrication requirements unique.

Monograde oil viscosities are indicated by a single number (e.g. 30) and are designed to cover a single requirement as classified by the SAE J300 classification system. The viscosity of monograde oils is exclusively measured at 100°c, and not specified for colder winter ‘w’ conditions.


Multigrade oils are used for more modern applications due to their versatile design, performing efficiently across a wide temperature range. Multigrade oils must meet two viscosity specifications. They are denoted with a w and two numbers, such as 5w30. The ‘w’ represents winter, and the number preceding it (e.g. 5) represents the oil’s viscosity in cold temperatures. Lower numbers indicate that the oil flows more easily in cold conditions, which is beneficial for quick engine lubrication during cold starts and fuel economy in cold conditions. The second number (e.g. 30) represents the oil viscosity at operating temperature. Higher numbers indicate thicker oil, which are suitable for high-temperature applications.

We typically group multigrade oil viscosities by the second number because this is the viscosity of the oil when it is at operating temperature and many oils will not ever be exposed to the lower temperature they are tested at. Multigrade oil viscosity is a combination of cold and hot temperature performance.

First Number
Tested at (Crank)
Tested at (Pump)

Makeup of Oil

Millers Oils is an independent blender of finished lubricants, produced by blending base oils and certain additives to create products that meet OEM specifications. Some of the main requirements include, wear protection, engine cleanliness, and defined temperature-viscosity properties. The properties of each blend depends on the base oil used, Mineral, Synthetic Based or Fully Synthetic, and the varying additives.


  • Viscosity improvers help oil to maintain its optimal viscosity under different temperatures and pressures.
  • Anti-wear agents such as ZDDP coat engine parts to protect them from friction and wear.
  • Corrosion and rust inhibitors prevent the formation of rust and corrosion on metal engine parts.
  • Detergents clean engine deposits and sludge that can reduce an engine’s efficiency and performance.
  • Dispersants remove contaminants and deposit them in the filtration system.

Manufacturer Requirements

Engine protection

One of the main reasons for selecting the correct oil viscosity is to ensure proper engine protection, especially during cold starts. Using an oil with a viscosity that is too high for the temperature means that the oil will not flow quickly enough to lubricate the engine and you may have difficulty starting your car. This can lead to reduced fuel economy and excessive wear which can cause significant engine damage over time. An oil with a viscosity too low for cold weather can result in inadequate lubrication, leading to similar problems.

Once your engine is running and reaches its operating temperature, the oil viscosity becomes equally critical. Using oil with the correct viscosity ensures that the engine’s moving parts are adequately lubricated, reducing friction and providing sufficient protection for the engine.

Fuel Economy

Selecting the right oil viscosity isn’t just about engine protection, it also affects fuel efficiency and engine performance. Oil that’s too viscous may create extra drag, reducing fuel efficiency which is usually noticeable through reduced mileage per gallon. While oil that’s too thin may not provide the necessary lubrication for optimal performance.

Drain Interval

OEM oil drain intervals vary greatly, depending on their own extensive research and testing. In most cases, the recommended drain interval is based on standard conditions and will need to be adjusted accordingly. For example, when operating in more challenging conditions, such as extreme temperatures or driving conditions which put strain on components, including stop-start driving.

Modern Consideration

Further to manufacturer recommendations, updated Government legislation and new requirements and specifications are also a factor in choosing the right oil viscosity.

Government legislation

As Government legislation regarding emissions and efficiency goals gets increasingly stringent, new lower viscosity, synthetic oils are becoming more popular to provide improved fuel economy. These newer oils are not backwards compatible with older specifications.

Emission control devices

Emission control devices (after-treatment systems) are used to regulate and eliminate the emission of potentially hazardous substances from engines. If you have a diesel car fitted with a diesel particulate filter (DPF) you must use a low SAPS (Sulphated Ash, Phosphorous, Sulphur) oil. Using a regular oil that is not suited to the emission control device can result in a blocked DPF.

Increased oil requirements

Modern engines need oils that are durable and last for extended mileage between services. But extended service intervals, thinner, synthetic oils and the greater use of smaller capacity turbo charged engines can all add up to higher oil usage. In modern engines with reduced oil sump capacity, a decrease in oil level can have more detrimental effects. This is because the oil has less time to cool down in the sump, resulting in hotter oil that will break down and oxidise more rapidly, leading to increased wear.

Making the Right Choice

If you’re struggling to find the right oil for your vehicle, Millers Oils WhichOil? Oil Checker Tool will provide a professional recommendation based on your vehicle requirements. There might be one or multiple brand ranges that could suit your vehicle’s engine based on the OEM recommendation. The choice depends on the driver’s specific requirements. Explore more about Millers Oils’ automotive brand ranges in this blog.

Similarly, your vehicle might also accommodate a range of viscosities or specifications set by the manufacturer. As a basic guide:

Thicker oils (with a higher second number) might be more suitable for:

  • Vehicles with a higher mileage, where a thicker oil can provide more protection
  • Engines that operate in high temperatures
  • Vehicles used in high performance environments
  • Older vehicles

Thinner oils (with a lower second number) might be more suitable for:

  • Vehicles needing better fuel economy as there is less drag on components
  • Engines that run in cold climates
  • Modern vehicles

However, this does not apply to every vehicle. Always consult the vehicle handbook and if you’re still unsure, contact Millers Oils Technical Support team for a recommendation.

Choosing the correct oil viscosity for your vehicle directly impacts engine protection, performance, and fuel efficiency. Making the correct choice is a small step towards preserving your vehicle’s health and longevity. To ensure your vehicle runs smoothly and stands the test of time, seek the advice of a knowledgeable professional.

Our in house technical support team can offer recommendations and advice, get in touch with them at: technical@millersoils.co.uk

Find which oils are suitable for your vehicle using our WhichOil? tool.

Buy your engine oil from the Millers Oils shop.

Or find your local distributor.