How to choose the motor oil right?

Last updated of 4 April 2025 by Maxime de Prinel

The choice of the suitable engine oil is essential to protect the engine, reduce fuel consumption and prolong the life of the vehicle. 5W30 and 5W40 oils are among the most commonly used, especially for modern vehicles. But which one to choose? In this article, we examine their specificities and help you make the best choice for your engine.

What is the viscosity of a engine oil?

The viscosity of an oil determines its ability to flow to different temperatures. It is expressed according to the SOE (Society of Automotive Engineers) classification, for example 5W30 OR 5W40.

• The first figure (5W) : Indicates the fluidity of the low temperature oil (W for «winter»). The lower than this figure, the colder is the cold oil and facilitates the beginning.
• The second figure (30 or 40) : represents the viscosity at high temperature (100 ° C at 150 ° C). The greater this figure, the more the oil maintains a certain thickness when the engine is hot.

Main differences between 5W30 and 5W40 oil

1. Hot viscosity
   • 5W30 : The oil is more fluid in a hot diet, which reduces the internal friction and promotes fuel saving.
   • 5W40 : The oil remains more often at high temperature, ensuring better protection in extreme conditions (heavy loads, high heat).
2. Fuel consumption
   • 5W30 : Thanks to a better fluidity, the strength force of less, which can slightly reduce consumption.
   • 5W40 : It can be slightly less efficient in terms of consumption due to its greater viscosity, but the difference remains minimal.
3. Thermal resistance
   • 5W30 : Perfectly suitable for temperate or cold climates and «normal» guide.
   • 5W40 : Ideal for warmer conditions, intensive use of the vehicle (high speeds, heavy loads, trailer) or if the manufacturer requires it.

5W30 Oil advantages

1. Fuel savings
   • Less friction inside the engine thanks to its fluidity.
   • It can contribute to a gain of a little percentage on consumption.
2. Cold the structures begins
   • His winter viscosity index (5W) allows him to circulate well in the engine from the beginning.
   • Particularly suitable in regions or winter periods.
3. Compliance with environmental standards
   • Many manufacturers recommend 5W30 to conform to anti-pollution standards (Euro 5, Euro 6, etc.).
   • Promotes the reduction of CO₂ emissions.
4. Versatility
   • Suitable for a large number of modern, petrol or diesel engines with or without particles filter (FAP).

5W40 Oil advantages

1. High temperature improved protection
   • Its higher viscosity (40) provides a lubricating film more often when the engine is hot.
   • Ideal in case of high environmental temperatures or intense engine stress.
2. Resistance and longevity of the engine
   • A more «disconcerted» oil in extreme conditions, limiting the wear of the internal parts.
   • It can be interesting for older engines or for those who are already consuming oil.
3. Less loss of oil in serious conditions
   • Thanks to its greater viscosity, it remains more stable in the face of extreme temperatures and oxidation.

1. Follow the manufacturer’s recommendations
   • The first criterion to consider is the recommendation of your vehicle brand.
   • Consult the maintenance manual or check the label under the hood / in the fuel port.
2. Take into consideration the climatic conditions
   • 5W30 : Rather adapted to temperate or cold regions.
   • 5W40 : Better suitable for very hot environments or if the engine is often stressed (sports driving, mountains, etc.).
3. Examine your driving style
   • Urban guide / short trips: 5W30 can be more interesting for its fluidity properties at startup.
   • Driving on a highway or a high load: 5W40 can provide additional protection against usury.
4. Take into account the age and mileage of the engine
   • On a recent engine, favor of the recommended oil, often 5W30.
   • On a high mileage engine, the 5W40 can help limit oil consumption thanks to its greater viscosity.

Maintenance and emptying advice

1. Exhaust frequency
   • Respect the recommended emptying intervals (on average every 15,000 to 20,000 km or once a year).
   • The oils so called «long life» can go up to 30,000 km, but also depends on the driving environment.
2. Regular control of the oil level
   • Check the level on the indicator at least once a month, especially before a long journey.
   • A lack of oil can seriously damage the engine.
3. Choice of certified oil
   • Check the ACEA (Association of European Automobile Producters Association) or API (American Petroleum Institute) on the container.
   • Choose oils that respond to the approval of the manufacturer (VW 505.00, MB 229.5, PSA B71 2290, etc.) if the vehicle requires.
4. Oil filter and global maintenance
   • Change the oil filter with each emptying to ensure optimal filtration.
   • Regular maintenance (candles, air and fuel filters, etc.) also contributes to the correct functioning of the engine.

The choice between 5W30 and 5W40 depends on the vehicle

The choice between an oil 5W30 or 5W40 It mainly depends on the specifications of the vehicle manufacturer, driving style and climatic conditions. The 5W30 oil stands out for its fluidity and can improve the saving of fuel, while 5W40 offers better high temperature protection and in more serious conditions. In any case, it is essential to respect the manufacturer’s recommendations and perform regular maintenance. An adapted oil associated with rigorous changes is the guarantee of an efficient and durable engine.