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Everything you need to know about motor oil

Whether you’re a die-hard petrolhead or you simply see your car as a means of getting from one place to another, it’s important to acknowledge just how important looking after your vehicle’s engine is. Anyone who owns or is the keeper of a car or van has a duty to take care of their vehicle’s engine, and to ensure performance and reliability as well as safety. One of the simplest and most effective ways of achieving this is to use motor oil of the best available quality.

The oil is what makes the engine work effectively and efficiently, so why wouldn’t you want to invest in the very best oil to keep it running smoothly? It may cost a little more in the short term, but the outlay is likely to be far less than the cost of having to repair – or even replace – the engine.

Why does it matter what oil I put in my car?

There are a number of ways in which using a good motor oil is key to the performance of a car engine.


An engine is made up of moving parts – much, in fact, like the human body. Take away the cartilage or synovial fluid from a joint, and your knee, hip or elbow bones would actually grind together. This is because they require lubrication, to prevent too much friction. In the body, this would cause pain and perhaps stiffness.

Lubrication of an engine is crucial because increasing friction also leads to temperature elevation, and an overheated engine can be dangerous as well as ineffective. Friction itself can also cause additional wear and tear to those parts that are rubbing together. This will eventually lead to damage, so once again the possibility of expensive repairs or replacement is raised. It could even be the case that replacing the car would be cheaper, once too much damage is done.


Even when friction is reduced by using a decent motor oil, friction still occurs between moving parts. This creates heat, but the oil insulates the engine against excessive rises in temperature that could adversely affect the process of combustion.

The chemical reactions that take place during combustion result in the formation of various substances such as sludge and other deposits. These can build up over time. Top quality engine oils contain additives that can attract these substances and draw them away from where they could cause damage. This can be achieved, for example, by holding them within a thick, viscous suspension.


Oxidation occurs because air is drawn into the car’s engine to facilitate combustion, and oxygen is one of the components of air. Oxidation leads to corrosion and rusting, and while oxidation cannot be prevented, using the right engine oil can protect the engine from its effects.

Which type of oil is best for my car?

There are two main types of motor oil – mineral or synthetic.


Mineral oil comes from under the sea and is made during oil refinery. This type tends to be cheaper than its synthetic counterparts.


Synthetic motor oils tend to be more costly because of the way in which they are created. While they do contain mineral oil, they have been specially engineered to last for longer. This means less frequent oil changes are required. Synthetic formulations also contain additives that are designed to improve system cleanliness. These oils also offer increased stability, especially at extreme temperatures.

How do motor oil viscosity grades work?

All types of oil seem to promise to maintain a cleaner and more efficient engine, so what do those letters and numbers on the bottle actually mean?


These indicate the oil’s viscosity rating – sometimes referred to as its weight. Viscosity measures the speed of oil flow. Thus the viscosity tells us how thick the formula is, and therefore how quickly it will flow. All motor oil is tested by The Society for Automobile Engineers (SAE), and it is rated between 20 and 60. So typical codes found on the oil bottle include SAE 20 or SAE 30 and so on.

5W-30 and similar codes

In countries with cooler climates, the label may have a code like 5W-30. This indicates that the oil has been SAE tested at lower temperatures. The oil is designed to be thinner when the engine starts, but once it gets going the oil will act just like on with an SAE 30 label.

If you’re still in any doubt, refer to your vehicle’s manual, which should suggest the right viscosity of oil for your specific make and model. You could also consult a qualified motor mechanic.

How often do I need to put oil in my car?

It used to be the case that motorists were advised to change the oil every three months or 3,000 miles, but modern cars can generally cope with longer between oil changes.

A lot of vehicles can travel for between 5,000 and 10,000 miles before an oil change is needed. This does vary accord to make and model, as well as how you drive. Travelling 5,000 miles between changes is thus a good general guideline, but for those whose journeys involve a lot of stopping and starting, more frequent oil changes are advisable. Once again, if unsure you can check your manual or speak to a mechanic.

Many modern cars will even let the driver know when it’s time to change the oil. Honda’s Maintenance Minder, for example, can monitor conditions within the engine so that oil changes are recommended when they are actually necessary, rather than in accordance with set maintenance intervals.

How much oil do I put in my car?

If you’re topping up the oil, then the traditional dipstick method is best. Make sure the engine is cool before using the dipstick to check the oil level. If it needs more, use a funnel to add some oil them wait a couple of minutes before checking the dipstick again. This ensures overfilling the engine is avoided. There will be sufficient oil once the level shows just under maximum. Performing an oil change, however, is a job best left to a qualified motor mechanic.

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