What kind of engine oil is right for your car?

Oil is the lifeblood of your car's engine. Without it, your car would cease functioning within minutes and your engine would likely sustain critical damage. Today's engine specifications are becoming increasingly strict, so choosing the right oil for your engine is now more important than ever.

Which engine oil should I use?

What can happen if you use the wrong engine oil?

Automobile engines are precision machines, so the manufacturer's engine oil specifications should be seen as requirements, not recommendations.

Using the wrong engine oil could:

• void the warranty on your car
• decrease fuel economy
• cause premature wear on engine components
• ultimately shorten the life of your engine

Your engine was designed to run on oil of a specific viscosity rating. If you don’t put this type of oil in your engine, your car won't get the gas mileage, emissions, and horsepower promised by the manufacturer. Using the wrong engine oil also absolves the manufacturer of repairing or replacing any damaged parts if your car is still under warranty.

Using the wrong oil will put extra stress on your engine's internal components. Oil that is too thick may not flow enough to fill some of the tighter spaces between engine parts. Oil that is too thin may fail to adequately coat all parts. Either outcome involves metal-on-metal contact which will wear components down prematurely. If left unattended, it could lead to severe and costly engine damage.

Engine oil specifications

It is important to know the intended application of the engine oil you are buying, and whether or not it meets current industry standards. There are two universally accepted specifications to look for on a bottle of motor oil: API (American Petroleum Institute) and ACEA (Association des Constructeurs Européens d'Automobiles).

The API classification consists of two main categories. There is S, which stands for "service" and encompasses cars, vans, and light trucks with petrol engines. There is also C, which stands for "commercial" and encompasses heavy-duty vehicles that run on diesel fuel. The oil may say “API SN Plus.” This indicates the oil's capacity for protection against low speed pre-ignition (a kind of super-knock). The oil's capacity for better fuel economy will be indicated by the phrase “Resource Conserving”.

The ACEA is the European equivalent of the API, and indicates an engine oil's performance with three categories: A for petrol, B for diesel, and C for catalyst compatible or low SAPS (sulphated ash, phosphorus and sulphur).

As engine technology has advanced and become increasingly specialized, a number of automobile manufacturers have developed their own approval and specification criteria for the oil to be used in their engines. The specification which the manufacturer recommends in the vehicle maintenance book is the most important specification to consider. These are listed on the oil can packaging under Original Equipment Manufacturer Approvals. For example, if you have a Volkswagen, and the maintenance book recommends adding oil with the specification "VW 507.00," look for this specification first on the label of an oil can. Looking at the ACEA and API specifications of oil is optional, while the manufacturer’s specification is crucial.

Viscosity grade

Engine oil is given a viscosity grade which indicates its flow rate – think of viscosity kind of like “thickness”. This is an important quality in engine oil, because in order to perform properly, it must be able to flow around engine parts and coat them at different temperature extremes. Because of this, most modern engines require multi-grade oils.

The Society of Automotive Engineers has developed the industry standard for grading oil viscosity. The grading format consists of a number followed by the letter “w” (which stands for “winter”) followed by another number (e.g. 5w30 or 10w40). The number preceding the letter “w” denotes how easily the oil will flow at 0ºC, while the number after the 'w' denotes the oil's ability to resist flow around 100ºC (the engine’s running temperature). The higher the number, the greater the viscosity.

This grading system tells you how easily the oil can flow at cold temperatures and how resistant it is to flow at hot temperatures. This is important, because when conditions are cold, the oil needs to be thin enough to flow to all the necessary engine components upon ignition. As the engine runs and the oil heats up, its viscosity decreases, but it must remain viscous enough to coat and lubricate the engine's components.

5w30 engine oil behaves like oil with viscosity grade of 5 when it's cold and like oil with a viscosity grade of 30 when it's hot. Likewise, 10w40 oil behaves like oil with a viscosity grade of 10 when it's cold and like oil with a viscosity grade of 40 when it's hot.

To find out which viscosity grade is right for your car's engine, consult your owner's manual.

Conventional, synthetic, synthetic blends, and high-mileage oils

There are four types of oil:

• Conventional engine oil: The product of refined crude oil and chemical additives, this is the most widely used engine oil on the market, and the most affordable.
• Synthetic engine oil: This is oil which has a more uniform molecular structure, and fewer impurities. It lasts longer and performs better in most cars, and is therefore more expensive. Careful, though – some cars may react poorly to it.
• Synthetic blend engine oil: A mix of conventional and synthetic, this oil balances affordability and superior performance. It’s a good option for vehicles which are hauling, towing, or off-roading
• High-mileage: This oil has a conventional base, but includes special additives designed for older engines, like antioxidants, detergents, friction inhibitors, and seal conditioners.

Each has its own advantages and disadvantages. Some engines may have only one option. You shouldn’t decide yourself which of these is right for your engine. Always consult your owner's manual to find out which is best for your automobile, and respect the car manufacturer specifications.

Diesel vs Petrol engines

Both petrol and diesel engines need engine oil to function properly. But their internal mechanics are fundamentally different, meaning that their lubrication needs differ.

• Diesel engines tend to require oils with a higher viscosity grade, because their components have greater clearance between them.
• Another key difference between diesel engine oil and petrol engine oil is the amount of additives. Diesel engines produce a lot more soot, which necessitates more detergents to keep the engine clean. Because of these additives, diesel engine oil tends to last longer between changes than petrol engine oil.

It is best practice to use engine oil according to its intended purpose. Always follow your manufacturer’s specifications and the recommendations in your owner’s manual when choosing diesel or petrol oil.