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How to pick the right engine oil for your car

There are many options regarding motor engine oil. This guide explains what it all means and what to look for when choosing oil for your car.

Initially, the best place to start is by checking your vehicle manual. Here, you should find the recommended weight of oil for your specific vehicle.

Understanding the label

There are three different standards used to denote the type of car engine oil:


This comes from the European Automobile Manufacturers’ Association (ACEA), and is an indicator of quality determined by two digits: a letter followed by a number (e.g. A1).

The letter

The letter stands for the engine type. A is for petrol engines and B for diesel engines in domestic vehicles. C indicates a vehicle with a light engine that has a particulate filter or catalytic converter, and E stands for commercial vehicles.

The number

This indicates the performance level you can expect from the engine oil. A higher number indicates a superior performance.

ACEA guidelines

Three categories are used by the ACEA’s 2016 guidelines. A3/B3, A3/B4 and A5/B5 are for 
petrol or diesel engines, while there are five categories – C1 to C5 inclusive – for those vehicles fitted with a device for pollution control. Finally, there are four categories for commercial vehicles. Two of these – E6 and E9 – apply only to vehicles with a pollution control device, while E4 and E7 are for those without.


This standard, as decided by the Society of Automotive Engineers (SAE), relates to the viscosity of the oil. The code on the oil packaging – such as 00w00 – comprises the following:

Left side

The first number on the left-hand side denotes the oil’s low-temperature viscosity. A lower number will offer a more efficient performance at a low temperature.

Right side

The number on the right hand side gives some idea of the product’s performance in high-temperature conditions. A higher number indicates an oil that can readily cope with elevated temperatures.


This standard is determined by the American Petroleum Institute, and categorises the engine oil according to set criteria; namely its properties of protection against oxidation, corrosion and wear, and its detergent and dispersive power. There are two letters on each bottle. The first will be S or C for petrol or diesel engines respectively, while the second is an indicator of performance. A letter later in the alphabet denotes a higher quality oil.


An engine oil’s viscosity indicates the resistance to flow. Motor oil becomes thinner as its temperature rises, and thicker as it cools down. The right additives can stop the oil becoming too thin. An engine oil may display one viscosity when it’s cold and a completely different one when it’s hot. A higher second number denotes an oil that is more resistant to thinning caused by high temperatures. This is generally a desirable property, as a thicker oil will usually seal better and thus maintain more effective lubrication between moving parts.

If an oil is excessively thick, the engine may demand extra energy. As more power is needed to turn the crankshaft, an oil that is too thick can make starting the engine more difficult, and this will require more fuel. Typically, 5W engine oils are recommended for cold weather conditions, although synthetic oils may be formulated so that their flow is even more effective during winter.

The second number used to determine the engine oil’s viscosity – like the 40 in 10W-40 – indicates that this oil will be thicker when temperatures are raised than with an oil that has a lower second number – such as the 30 in 10W-30.

The key is to use the correct engine oil viscosity as directed by the owner’s manual.

Synthetic versus Mineral Engine Oils

Premium Mineral Oils

This is the latest motor industry standard. The major oil brands will usually offer several viscosities. Car manufacturers often suggest the use of a 5W-20 or 5W-30 oil, especially for low temperature conditions. They may also state a 10W-30 option for use in higher temperatures.

Fully Synthetic Oil

Fully synthetic engine oils are usually designed specifically for high-tech engines. Such oils often provide a superior, enduring performance in all areas, from the viscosity index to protective properties. They generally offer enhanced flow at all temperatures. However, such oils are costly and not every vehicle’s engine requires this level of performance.

Synthetic Blends

These combine synthetic and mineral oil, and are formulated for high-demand conditions. Such oils tend to be less volatile, which reduces evaporation and thus increases fuel economy. These blends tend to be less pricey than fully synthetic oils.

High Mileage Oils

The vehicles sold today are made to last longer, and thus are more likely to clock up a higher mileage. There are thus engine oils that are specially formulated for such vehicles. Sometimes older vehicles are prone to oil leaks, as the seals have lost flexibility and developed cracks, due to hardening over time. Higher-mileage engine oils contain conditioners that can maintain and even restore the shape and flexibility of the seals.

High-mileage vehicle owners may also report a loss of performance, and notice that the engine runs less smoothly than before. Higher-mileage oils have higher ranges of viscosity – even when this isn’t indicated by the viscosity index – as they often contain additives to improve the viscosity index. They might also have superior additives that prevent corrosion and wear.


Additives are used by engine oil manufacturers to improve and maintain performance. The high operating temperatures of vehicle engines are combined with combustion byproducts, rust, moisture, corrosives, particles and oxygen, and this leads to an accumulation of deposits and sludge. The right additives can maintain good levels of lubrication, as well as minimising the build-up of sludge and particles that could damage the engine.

Helpful additives that oil formulations may include are detergents for cleanliness, viscosity index improvers that reduce thinning, pour-point depressants to ensure flow, dispersants that hold solid particles in a suspension, anti-wear agents to protect the moving metal surfaces, foam inhibitors that disperse foam bubbles, corrosion inhibitors that preserve metal parts, and friction modifiers that may improve fuel economy and antioxidants, which are necessary for those high engine temperature operations demanded for curbing emission levels.


Picking the right engine oil for your car is really a matter of sticking to the manufacturer’s guidelines, but knowing a little about why the oil’s features matter can go a long way.

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