After petrol or diesel, oil is the other critical car consumable that will leave you stranded if you don't top it up. Our car engine buyer's guide explains the importance of oil, which one you'll need and some top tips to make sure your car enjoys a long life with fewer breakdowns!

What is car engine oil?

Car engine oil is the same stuff that's sucked out of the ground by oil rigs, except it's refined from the basic black stuff into something much thinner and smoother that includes additives that help to clean and reduce wear and tear on internal components.

What does car engine oil do?

Motor oil is the lifeblood of your car. In certain sections of your engine, oil creates a layer between the moving parts that helps to prevent friction. Too much friction means the components will wear down faster, causing expensive damage to your car. This is why it's incredibly important to make sure your car has enough oil. If you do let your car run out of oil, then you run the risk of engine overheating, damage to expensive components, and eventually complete engine destruction!

What's in motor oil?

Unsurprisingly, the vast majority of engine oil is, well, oil! You'll also find things like detergents, which clean the spaces between moving parts and nooks and crannies where grime and old oil can build up, anti-wear (AW) additives that prevent certain materials from wearing out sooner, dispersants that prevent oil and other waste materials from clumping together, and viscosity improvers that help the oil to flow better, especially when it's cold.

Why does engine oil run out?

Thanks to the extreme heat and pressures inside car engines, oil degrades over time and becomes less able to protect your engine, so you'd eventually have no oil left in your engine at all if you didn't top up regularly. If your car is reaching a ripe old age, then the seals and gaskets may not be as tight as they once were, letting small amounts of oil leak out too. IMPORTANT: if your car is leaving puddles of oil on your driveway or you have to top up every few weeks, visit a mechanic as soon as possible, as oil leaks can lead to bigger, more expensive problems as time progresses.

What do the numbers mean on the oil container?

When you go to buy motor oil, you'll notice that there's a code on each container. The numbers relate to the oil's viscosity (how well it flows around the engine) in warm and cold temperatures, which is an indicator of how it'll perform depending on the outside temperature.

Thanks to the hugely varying temperatures in the UK, the oil we put in our cars is different to the oil you'd use in warmer countries. This is why we use the dual numbering system (known as 'multi-grade' engine oil), rather than single-grade varieties used in hot or tropical climates.

Which oil do I need for my car?

You can use our brilliant car engine oil finder, which will show you exactly which oil you need using just your car's registration number! We also recommend checking your owner's manual to get the right oil for your car, but you can always ask a Halfords colleague if you're in-store and they'll let you know which one you need!

When should I top up my oil?

This one really depends on how many miles you do and in what conditions. If you do a long daily commute, then you should check and top up if necessary every few weeks. You can check how much oil your car in the reservoir (where oil sits when the engine isn't running) by checking the dipstick or for vehicles without a dipstick, use the on-board vehicle information display - if the oil level is low, then you'll need to top up.

Get a full breakdown of how to check and top up your oil with our how to top up your engine oil guide.

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

Knowing how much oil to put in your car depends on two factors: Your car's engine oil capacity and how much oil there currently is. Check your owner's manual to get an idea of how much oil your car can hold and top up accordingly. Another way to do this is to check the oil level using your dipstick or for vehicles without a dip stick use the on-board vehicle information display. If the oil is halfway between the first line and the second line, then you'll need to put about half a litre in. If it's just over the first line, put a full litre in. Please note that these amounts refer to the average family car, and you may need to add less or more oil depending on if the vehicle has a smaller or larger engine oil capacity or a high performance engine. If you have any leftover oil, then this can be stored safely in your garage in an upright manor and away from heat sources ⁄ direct sunlight for up to 5 years. Always check the best before date before topping up too.

What is an oil change?

Just topping your oil up now and again isn't enough to keep your engine in top shape. An engine oil change involves completely draining the engine of oil and replacing it with brand new oil. You can do this yourself, but it can get very messy, so we'd recommend visiting a Halfords technician. They'll also be able to provide a full engine tune-up and potentially add further fluids that will help to clean degraded oil out of your car engine.

If your car needs a once over or you don't fancy getting oil all over your driveway, then just pop into your local Halfords store! Our auto experts will happily check and top up your oil.