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Common myths about engine oil

It’s likely that at some point, for example when taking your car for an MOT or service, you’ve heard something regarding engine oil that isn’t true – possibly even from the mouth of a mechanic. The reality is that engine oil varies, and how you use it can also differ according to factors such as the make and model of the vehicle.

It’s important to know what is a myth and what is true, as in extreme cases, incorrect use of oil can even ruin your car’s engine. Whether you leave oil changes to your mechanic or you prefer to change the oil yourself, clarifying common misconceptions regarding engine oil can help you to protect your vehicle.

So, let’s examine some regularly cited claims and advice about engine oil, and determine whether they should be taken seriously:

Change the engine oil every 3,000 miles

Many people believe that engine oil should be changed every 3,000 miles. However, this is misleading, as vehicle (and thus engine) specifications differ. One vehicle manufacturer might recommend changing the oil every 3,000 miles, while others state 5,000 or 7,000. Rather than using guesswork, it’s best to check the manual, or keep an eye on the car’s diagnostic system, which should advise when a change is due.

Change the engine oil before going on a long journey

It may be wise the change your engine oil before a long trip if your vehicle would be due a service while you were away. If you had a recent service, you can wait until you return. In this case, monitor the oil dipstick, and if the level drops below minimum at any time during the trip, top up the oil.

Change the engine oil if it’s black

Brand new engine oil is amber in colour, but if it is no longer amber, that does not necessarily mean it must be changed. Modern oils are formulated to clean the engine, so the darkening of the oil means things are working as just as they should. The oil will absorb contaminants and dirt as part of this process. Thus, engine oil being black in colour is no cause for alarm; in fact it means the dirt particles will not contaminate the engine.

Once you use synthetic oil, you must always use it

Synthetic engine oil is prized in part because of its resistance to extreme temperatures, as well as its ability to cleanse. The continuous use of synthetic oil could thus improve the performance of your engine. Still, this does not mean that switching between synthetic and mineral oils will do any harm to the engine.

You can alternate between the two types of oil, assuming the viscosity is the same. Simply check the requirements of your specific make and model of vehicle and select your oil accordingly.

Synthetic oil can cause leakage

You might occasionally hear this myth from the owners of older cars. This is because when motorists first started using synthetic oil, in the 1970s, this did occur. Synthetic engine oils were initially known to cause shrinkage in the engine seals, which could lead to oil leaks.

However, there has been a drastic reduction in the incidence of leaks, as many advances in technology have been made since the 1970s. The synthetic engine oils available today do not cause seal shrinkage and, as a result, leaks are no longer associated with synthetic engine oil.

High-quality engine oil will leak from older engines

There exists a common myth that car owners should expect oil leaks with older engines. This is a misconception, as top-quality engine oils are throughly tested. If leaks occur, the cause is more likely to be malfunctioning seals.

You can change the oil and not the filter

As the engine oil and filter work together, it is advisable to change the oil filter at the same time as changing the oil. This prevents any dirt or contaminants caught in the old filter from combining with the new oil. This could cause damage to the engine, and would at least mean that the new oil would become contaminated almost instantly.

The ‘W’ on the packaging stands for weight

It’s an easy assumption to make, but this simply isn’t true. The ‘W’ actually stands for winter. The viscosity of oil varies according to temperature, and the two numbers preceding the ‘W’ denote the viscosity temperature rating of the engine oil. As a general guideline, a lower number indicates a more viscous oil, and a higher number a less viscous oil. Once more, it is prudent to check the car manufacturer’s instructions when choosing the correct engine oil.

Thicker engine oil is better

Not necessarily. In fact, thinner engine oils often offer the best levels of viscosity, and are thus frequently the most compatible with car engines. Thinner oils tend to lubricate more effectively and this will help to reduce contamination within the engine. The use of a thinner oil may also reduce friction between moving parts, while requiring less energy to inject it through all parts of the engine.

A higher-quality engine oil is not needed if the vehicle runs well

On the contrary, poor-quality engine oils have the potential to harm a the car’s engine. They may be cheap to buy, but a ruined engine is very costly to rectify. Low-quality engine oils may contain high levels of contaminants, and these can in turn decrease the oil’s lubrication capabilities. As time goes by, the repeated use of inferior engine oil may mean you need to spend more on maintenance and repairs, and it could even result in engine damage. Conversely, using a high quality engine oil will improve the performance and reliability of your vehicle.

Sludge build-up is caused by all engine oils

The build-up of deposits – such as unused fuel and coolant leakage – within an engine can cause sludge. High-quality engine oils are capable of clearing such contaminants from the engine, holding them instead within a suspension. An obvious build-up of sludge could actually be caused by using low-quality engine oil.


The best advice regarding the use of engine oil is to refer to the directions provided by the manufacturer of your vehicle, as these will vary according to make and model. Engine oil should thus be chosen and changed as per their precise recommendations.

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