Your Guide to Servicing: Choosing Car Engine Oil

With choice at the motor factors being so utterly bewildering, you might be asking 'What engine oil do I need?' Here in our car oil guide, Rob Marshall advises on the importance of buying the correct engine lubricant. Read on for all you need to know about engine oil explained...

WHAT DOES ENGINE OIL DO?

Ultimately, motor oil separates your engine from a very large repair bill. Without lubrication, an engine will run for less than a few minutes, before being severely damaged, sometimes beyond repair.

Allowing the oil level to run too low, or not changing it on time, can also shorten engine life dramatically and may even reduce the value of your car, because the oil performs several critical functions:

1. LUBRICATION: Engine oil provides a film between the moving metal parts, to prevent them from contact, overheating and failure.

2. REMOVING HEAT: The lubricant transfers the intense temperatures, including that of combustion, and allows them to be dissipated.

3. HOLDING DEPOSITS: Engine oil retains combustion accumulations, such as acid and soot, in suspension, thus keeping the engine clean internally, which is why oil changes from a golden hue to black. Many years ago, when oil development was in its infancy, these deposits would build up within an engine, calling for it to be dismantled for ‘de-coking’ purposes.

4. PREVENT CORROSION: The many moving parts within an engine have to be protected from rust, meaning that acids require neutralising and moisture has to be kept away from the vulnerable areas.

5. HYDRAULIC MEDIUM: Certain engines use the pressurised oil supply to operate specific functions, such as hydraulic tappets or the camshaft chain tensioner.

IS A PETROL OR DIESEL ENGINE OIL TOP-UP NECESSARY?

Many owners are unaware than an engine tends to consume some oil naturally but the degree at which it does so depends on not only the engine’s condition but also its design.

Some types from certain manufacturers, especially high-performance power-plants, may need the engine oil to be topped-up between oil change intervals.

Most modern cars possess a fascia-based low level warning facility, which must be attended to immediately. Yet, owners should not rely on this and the oil level must be checked manually every week, using the dipstick.

Note that you should never fill an engine past the ‘maximum’ line on its dipstick. Extensive engine damage could result and topping-up should be performed judiciously, with the car standing on level ground and the dipstick markings being checked constantly, prior to more oil being added.

Note that the dashboard’s red oil level lamp indicates oil pressure failure, not necessarily a low level. Should it illuminate when driving, the engine should be stopped immediately.

When topping-up the oil, add a little bit at a time and wait a few minutes for it to travel to the sump, prior to checking the level with the dipstick.

Oil in engine

When you carry out a car engine oil check, never allow the oil level to fall below the minimum mark on the dipstick and do not add too much to put the level above the maximum mark.

WHY IS AN OIL CHANGE NEEDED?

With the passing of both time and mileage, engine oil loses its protective qualities and becomes saturated with contaminates. It needs to be drained and replaced with new oil to the correct grade and specification.

Ignoring the specified oil change interval by extending it can increase not only fuel use but also tailpipe emissions, while power output can become restricted. Obviously, engine life can be shortened considerably as well.

Oil change

Engine oil becomes contaminated, which is why it changes colour from a golden hue to either brown or black. Note that used engine oil is carcinogenic and regular skin contact should be avoided. Dispose of any discarded oil at your local council household recycling depot.

HOW DO I CHOOSE THE CORRECT ENGINE OIL?

Unfortunately, there is no one-type-fits-all engine oil and so, if you're wondering what oil should you put in your car, you have to tailor your choice to the specifications that are provided in your car’s handbook or service schedule.

There are many car oil types. When buying engine oil, ignore the branding on the container and look closely for the following specifications:

Viscosity

This is how thick/runny the engine oil is at certain temperatures. Multigrade oil viscosity is expressed as an SAE of two figures that are separated by a W, which denotes the winter or the ‘cold start’ rating. Generally speaking, 20W50 is the most common oil grade that is suitable for most pre-1980 classic cars.

The thinner 10W40 was the most common until the early 2000s and 5W30 is currently the most in-vogue viscosity for newer engines. Never use oil of an inappropriate viscosity for your engine.

Specification

The European API rating is the most relevant for modern cars sold in the UK, at the time of writing. The A grade is for petrol, B denotes diesel engine oil and C is for modern diesel engines with particulate filters (DPF).

Note that diesel cars fitted with DPFs require diesel engine oil with low levels of SAPS (sulphated ash, phosphorus and sulphur), to reduce the chance of the particulate filter clogging with deposits from the engine oil that is burnt naturally.

The API (American Petroleum Institute) rating is also quoted, which uses the prefix ‘S’ for petrol and ‘C’ for diesel, followed by another letter than indicates the quality.

Mineral, synthetic or semi-synthetic oil?

Generally speaking, mineral oil is only suitable for basic, older, non-turbocharged engines. However, it is inadvisable to use fully-synthetic oil in the engine of a classic car that was designed for mineral oil.

Synthetics are man-made additives that enhance the oil’s tolerance of extreme heat. Some older engines, especially turbo-diesels, thrive on semi-synthetic oils but most modern vehicles tend to demand fully synthetic, which tends to be more expensive. Never try and cut costs by using either a mineral or a semi-synthetic oil in an engine where synthetic is specified.

All engines have their own requirements for oil viscosity and quality. Look beyond the claims on the packaging and focus on the relevant engine oil grades. This oil is of a ‘high-performance’ type but the packaging does not indicate if it is mineral, synthetic or semi-synthetic.

The thickness (or viscosity) of an engine oil is essential and is expressed as the SAE figure. This multigrade oil is formulated specifically to meet the needs of most classic car engines and its quality is reinforced by it being able to either meet or exceed various API standards.

Oil quality can be expressed as either American API or the European ACEA standards, or both. For the latest diesel engines, the European rating tends to be more prominent.

Oils for the latest models tend to boast about meeting manufacturer requirements. Yet, do not treat them as proof that they are suitable for your car. Such specifications might be incompatible with your engine and you should treat the ACEA/API quality and SAE viscosity grades as the more relevant.

WHAT ABOUT OIL ADDITIVES?

Generally speaking, if your engine is in good condition, it is more important to use the correct oil and change it regularly, than to spend money on an additive that could work against the chemical properties within the oil.

However, it is worth considering the use of an oil flush additive, just prior to an oil change being carried out.

ARE OTHER OILS SIMILAR?

Additional fluids, such as the types used in gearboxes, differentials and even the braking system, possess their own grades and you should ensure that you buy the correct type that is stipulated by the car manufacturer.

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