The key difference between 5W30 and 5W40 engine oils is their high-temperature viscosity. 5W30 is thinner at operating temperature, offering better fuel efficiency, while 5W40 is thicker, providing enhanced engine protection under high loads, long-distance driving, or hot climates. Both have the same cold-start rating (5W) for smooth lubrication in low temperatures.
Choosing the right engine oil is one of the most important decisions for maintaining your vehicle’s performance, longevity, and efficiency. Among the most commonly used oils are 5W30 and 5W40. While both oils are multi-grade and share some similarities, they have distinct properties that make them suitable for different engines, climates, and driving conditions. Understanding the differences between them is essential for anyone who wants to optimise their vehicle’s performance and protect their engine from wear and tear.
This comprehensive guide explains the characteristics of 5W30 and 5W40 oils, their advantages, ideal usage conditions, and how to choose the right one for your car. Whether you drive a petrol or diesel vehicle, a city commuter, or a long-distance traveller, this guide will help you make an informed decision.
Understanding Engine Oil Ratings
Engine oils are categorised by a viscosity grade, which indicates how thick or thin the oil is at certain temperatures. Viscosity is crucial because it determines how well the oil lubricates engine components, protects against wear, and maintains performance under varying temperatures.
The numbers in oil grades such as 5W30 and 5W40 describe the oil’s viscosity:
- The first number (5W): Represents the oil’s viscosity at low temperatures (winter). “W” stands for winter. Lower numbers indicate better flow in cold conditions, improving engine start-up and lubrication during winter.
- The second number (30 or 40): represents the oil’s viscosity at high temperatures (100°C, engine operating temperature). Higher numbers indicate thicker oil at operating temperature, which can provide better protection under heavy loads or high temperatures.
Key Differences between 5W30 and 5W40
While both oils share the same winter viscosity rating (5W), the main Difference lies in the high-temperature viscosity:
Viscosity at Operating Temperature
- 5W30: Thinner at operating temperatures (SAE 30), providing lower resistance and easier flow through the engine. Ideal for engines that require lower high-temperature viscosity.
- 5W40: Thicker at operating temperatures (SAE 40), offering more robust lubrication under high stress, high load, or high-temperature conditions.
The Difference in thickness at operating temperatures impacts engine protection, fuel efficiency, and performance in different driving conditions.
Fuel Efficiency
Because 5W30 is thinner at operating temperature, it provides less internal friction, which can improve fuel economy. Vehicles designed for 5W30 often achieve slightly better fuel economy than those designed for 5W40.
Engine Protection
5W40 provides better protection under extreme conditions, such as:
- High-speed driving
- Heavy loads (towing or commercial use)
- Hot climates
Thicker oil maintains a protective film between moving engine parts, reducing wear and prolonging engine life in demanding conditions.
Cold Start Performance
Both 5W30 and 5W40 offer excellent cold-start performance due to their “5W” ratings. This ensures easy engine start and rapid lubrication at low temperatures, helping protect components from cold-start wear.
Suitability for Modern Engines
Many modern vehicles, especially those with turbochargers or high-performance engines, are designed with specific oil requirements:
- 5W30: Often recommended for vehicles prioritising fuel efficiency and meeting Euro 6 emissions standards. Common in newer petrol and diesel engines.
- 5W40: Commonly used in older vehicles, high-performance engines, or those operating in warmer climates where additional high-temperature protection is beneficial.
Advantages and Disadvantages
5W30 Advantages
- Improved fuel economy due to lower high-temperature viscosity
- Compatible with most modern engines
- Excellent cold start protection
- Reduces emissions by maintaining optimal engine efficiency
5W30 Disadvantages
- May provide less protection in extreme heat or heavy-load conditions
- Not ideal for high-performance or older engines requiring thicker oil
5W40 Advantages
- Superior protection in high temperatures and under heavy loads
- Excellent for long-distance driving or towing
- Reduces wear in older or high-performance engines
5W40 Disadvantages
- Slightly reduced fuel economy due to higher internal friction
- It may not be necessary for all modern engines, especially those optimised for 5W30
Factors to Consider When Choosing Between 5W30 and 5W40
Manufacturer Recommendations
Always refer to your vehicle manufacturer’s manual. Manufacturers specify oil grades based on engine design, emissions systems, and expected operating conditions. Using the recommended oil ensures warranty compliance and optimal engine performance.
Driving Conditions
- City Driving: Frequent stop-start driving benefits from thinner oils like 5W30, which circulate more easily.
- Highway or Long-Distance Driving: 5W40 may offer better protection under sustained high temperatures.
- Towing or Heavy Loads: 5W40 is preferable for extra protection of engine components under stress.
Climate
- Cold Climates: Both 5W30 and 5W40 perform well thanks to their “5W” ratings, ensuring smooth cold starts.
- Hot Climates: 5W40’s higher high-temperature viscosity offers better protection against heat-related wear.
Engine Age and Design
- Modern Engines: Often optimised for 5W30 to balance fuel efficiency, emissions, and performance.
- Older or High-Mileage Engines: May benefit from 5W40’s thicker high-temperature film to reduce wear.
Synthetic vs. Semi-Synthetic
Both 5W30 and 5W40 are available in fully synthetic or semi-synthetic variants:
- Fully Synthetic Oils: Offer superior temperature stability, longer oil change intervals, and better protection under extreme conditions.
- Semi-Synthetic Oils: More affordable, suitable for standard driving conditions, and provide adequate protection for most engines.
Choosing the right type depends on your vehicle, driving style, and budget.
Oil Change Intervals
Oil change intervals vary by vehicle and driving conditions. Modern engines using synthetic oils may allow changes every 10,000–15,000 miles, while older engines may require more frequent changes. Always follow the manufacturer’s guidance. Using high-quality oils from 123 Oil ensures your engine receives the protection and longevity it needs.
Common Misconceptions
“Thicker oil is always better.”
Not necessarily. Oil viscosity must match manufacturer specifications. Using oil that is too thick can reduce fuel efficiency and harm modern engines designed for thinner oils.
“5W40 is better for all conditions.”
While 5W40 offers extra high-temperature protection, it is not always necessary and can reduce fuel economy in engines designed for 5W30.
“I can switch grades anytime.”
Switching between 5W30 and 5W40 is possible if the engine is compatible, but always consult your vehicle manual or a qualified mechanic before doing so.
Tips for Maintaining Engine Health
- Regularly check oil levels and top up as needed
- Use the correct grade recommended by the manufacturer
- Choose high-quality oils to protect engine components
- Monitor for unusual engine noises or warning lights
- Schedule timely oil changes to maintain optimal performance
Conclusion
Understanding the Difference between 5W30 and 5W40 is vital for every diesel and petrol vehicle owner in the UK. Both oils offer excellent cold-start protection and are multi-grade, but their high-temperature viscosities determine their suitability for specific engines and driving conditions. 5W30 is ideal for modern engines prioritising fuel efficiency and lower emissions, while 5W40 provides superior protection for high-performance, older engines, or vehicles operating in hotter climates or under heavy loads.
Selecting the right oil ensures optimal engine performance, longevity, and compliance with manufacturer specifications. Using high-quality oils from reputable suppliers, 123 Oil, guarantees your vehicle receives the protection it needs while maintaining efficiency and reducing environmental impact.
Whether you are a city commuter, a long-distance traveller, or a towing enthusiast, knowing the differences between these oils helps you make informed choices and maintain your engine in peak condition.
Frequently Asked Questions
The main Difference is high-temperature viscosity. 5W30 is thinner at engine temperature, while 5W40 is thicker, offering extra protection under stress.
Only if your vehicle’s manufacturer allows it, check the manual or consult a mechanic, as switching grades may affect fuel efficiency and engine performance.
5W30 generally offers better fuel economy due to lower friction at operating temperatures, whereas 5W40 prioritises engine protection over efficiency.
5W30 generally offers better fuel economy due to lower friction at operating temperatures, whereas 5W40 prioritises engine protection over efficiency.
You can purchase premium oils from retailers and trusted suppliers like 123 Oil, ensuring ISO-certified quality for your engine.
