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What is the difference between C2, C3 and C4 oil?

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Among other activities, the Association des Constructeurs Européens d’Automobiles (ACEA), also known as the European Automobile Manufacturer’s Association, defines engine oil specification. Known as ACEA Oil sequences, these classifications are regularly updated to remain current. While ACEA doesn’t approve oils, it sets standards lubricant manufacturers must meet when creating products.

ACEA specifications use letters of the alphabet to classify different engine oil types. For example, oils for passenger cars fall into the A/B category while C class engine oils are catalyst compatible products. Classifications are divided further using numbers to show suitability for different engine requirements.

C class engine oils are categorised by HTHS viscosities and SAPS limits. Below, we look at these qualifiers and C2, C3 and C4 engine oils in detail and what makes them differ from each other.

Understanding HTHS and SAPS

HTHS (high temperature high shear) is the viscosity of an engine oil at a temperature of 150°C (considered a high temperature in engine compartments) and a velocity gradient of 106*s-1 within a set measuring vessel. The HTHS value provides information about the fuel efficiency properties of an engine oil and the potential fuel savings that can be achieved with the oil.

An HTHS value is a measure of the engine oil viscosity under demanding driving conditions. Established limit values are designed to ensure that even in high-performance engines, the oils can continuously provide a tear-resistant lubricating film within the bearing area at both high speeds and high temperatures.

SAPS (sulphated ash, phosphorus and sulphur) content is necessary to help keep the engine oil’s Total Base Number, resist thermal breakdown and viscosity shear, and protect against wear, oxidation, deposit formation and corrosion.

Sulphated ash is the metallic content remaining after engine oil combustion. These particles are mostly formed by the engine oil’s detergent and anti-wear additives. As sulphated ash deposits aren’t combustible, regeneration doesn’t remove them, leading them to collect in the diesel particulate filter (DPF) or other such device used for exhaust aftertreatment.

When sulphated ash becomes too concentrated, it can block the filter, causing loss of engine power and functionality. It can also cause formation of deposits on internal engine components, which can result in engine damage.

Phosphorus is an engine oil additive that provides anti-wear protection and enhanced oxidation resistance. Phosphorus creates a thin layer over metal surfaces, which limits friction between metal parts. Sulphur is another additive that promotes engine cleanliness, but also supplies antioxidant and anti-wear protection.

However, regardless of these favourable characteristics, when sulphur and phosphorous concentration is too high, it can poison and cause damage to a vehicle’s catalytic converter and lead to a build-up of sulphated ash within the engine and DPF.

ACEA C2 engine oils

C2 class lubricants are considered stable and stay-in-grade engine oils. They have a mid SAPS level and are used as catalyst-compatible oil in vehicles with all modern aftertreatment system types and engines in petrol-powered passenger cars and light duty vans, as well as Direct injection (DI) diesel engines that are designed to use low viscosity oils with a minimum of 2.9 mPa*s HTHS viscosity.

ACEA C3 engine oils

C3 class lubricants are also stable stay-in-grade oils with mid SAPS level, for use as catalyst compatible oils with modern aftertreatment systems and high-performance DI diesel engines. However, they are suitable for engines designed to use oils with minimum HTHS viscosities of 3.5 mPa*s.

ACEA C4 engine oils

C4 engine oils are also formulated for engines that use oils with 3.5 mPa*s HTHS viscosities and are stable catalyst-compatible engine oils classed as stay-in-grade. However, where they differ from both C2 and C3 engine oils is that they are low SAPS level oils.

Always consult your vehicle user manuals to ensure you select the correct oil for your vehicle.


At Oil-Store, we have a variety of C2, C3 and C4 engine oils available from a wide range of brands, including Morris, Shell and Fuchs. Shop the range today.

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