Understanding your car's oil

What does oil do?

Your oil is, literally, what keeps your car running smoothly. It keeps all the different parts of your engine lubricated so that they work properly and don't start rubbing against each other and damaging each other. The oil is stored in a container called a sump. When the engine is in use, the oil flows around the engine, passing through a filter which removes impurities and particles.

What oil do I need for my car?

There are three different types of oil:

Conventional or mineral oil: This is the cheapest kind, made from crude oil, and manufacturers don't usually recommend it nowadays as it doesn't normally have the engine-cleaning detergents that modern cars need. The range of temperatures at which it works well is also quite limited. That said, it's fine for older engines in mild climates.

Synthetic oil: This is better quality than conventional oil: it performs well in a wide range of temperatures and conditions and can withstand the engine being worked hard, for example if you stop and start your car a lot, often tow heavy loads or drive a sports car. It also cleans the engine as it is pumped around, and helps to reduce fuel consumption. It is also more expensive.

Synthetic blend or semi-synthetic oil is a mixture of synthetic oil and conventional oil, so it protects your engine better than conventional oil, and is not as expensive as synthetic oil.

What about those weird numbers?

Different oils are manufactured with different viscosities (thicknesses) to make them suitable for different climates – oil thickens and flows more slowly in cold weather – and different engine types. The numbers on a can of oil, like 5W30 or 10W40, tell you about this. Most oils will have two numbers separated by a W, which stands for winter, though some just have one number, for example 20W.

The first number shows you how well the oil flows at cold temperatures, and the second tells you how well it flows at hot temperatures. The lower the number, the better the oil flows. But although newer engines tend to need better flowing oils, older engines, or ones which have done higher mileage, may work better with thicker, more viscous oils. Your manufacturer will tell you what kind of oil your car needs and you should follow their recommendation.

If your car has a diesel particulate filter then you must use low SAPS oil, also called low ash oil, or you risk blocking the filter. There are three kinds, C1, C2 and C3 – again, check your manual to see which kind your car needs.

Manufacturers' own oil grades

In addition to all this, many manufacturers have their own accredited oils. These are usually long-life oils, which can last for up to two years or 18,000 miles between changes, which is good because it means your car can have longer service intervals. Again, your handbook or your car dealer will be able to tell you if you need a manufacturer-approved oil.

How often do I need to check my oil level?

The AA recommends checking your oil level fortnightly, and again before a long journey. However, you might need to increase that to weekly if you do a lot of miles or have an older car.

To check your oil, you should park, switch off the engine and wait for three or four minutes until the engine has cooled. Pull out the dipstick, use a cloth to wipe the oil off it, push it back into its tube, then pull it out again and check the oil level. It should be between the upper and lower marks. If it is below or almost below the lower one you should top up your oil. Put the dipstick back into its tube, open the oil filler cap, pour some oil in, replace the cap and after 60 seconds check the dipstick again.

Oil becomes less effective over time, so your service will usually include an oil and filter change.

What should I do if my oil light comes on?

Never drive your car with the oil light illuminated. Low oil could lead to catastrophic engine failure in a very short amount of time. Pull off the road and turn the engine off as soon as possible. Check the oil and top it up if necessary; if the oil isn't low, or the light stays on or comes back on soon after you've topped it up, or you can see signs of a leak, you need to call your garage or breakdown service.

At Carwise Group, we sell a huge range of used cars – and our in-house technicians have been trained to perform car services and repairs on a wide variety of makes and models too. So if you'd like your car engine oil explained, checked and topped up, call us at whichever of our dealerships is most convenient for you – you'll find us in Harlow and Maidstone.