Oil Change Intervals: How Often Your Car Really Needs One

Engine oil is the cheapest insurance policy an engine has, and the oil change is the maintenance job most often either overdone on bad advice or left far too long on optimism. The right interval is not a folk rule, it is written in the handbook for your specific car, and it depends as much on how you drive as on how far.

How often should you change your engine oil?

Change the oil and filter at the interval in your handbook, which for most cars sold in the UK is somewhere between 10,000 and 20,000 miles or every 12 months, whichever arrives first. The time limit matters as much as the mileage, because oil degrades whether the car is used or not.

The 3,000 mile figure that appears all over the internet is imported American advice from the era of mineral oils and looser engine tolerances. It does not reflect what modern fully synthetic oil is capable of, and it does not match what any manufacturer selling cars in the UK specifies.

Where you should shorten the interval:

• Mostly short journeys where the engine rarely reaches full operating temperature.
• Stop start urban driving, heavy traffic or a lot of idling.
• Towing, roof boxes, heavy loads or repeated hill work.
• Track use or sustained high revs.
• A high mileage engine that is starting to use oil.
• Any engine known for oil related problems, such as timing chain wear on certain small capacity turbo petrols.

Fixed servicing or long life servicing: which should you use?

Many manufacturers offer two schedules, and the one your car is set to makes a large difference to how often the oil gets changed.

A long life schedule uses a service interval indicator that monitors mileage, engine temperature, journey length, revs and oil condition, then tells you when a service is due. It works well for a rep doing 25,000 motorway miles a year. It works badly for a car doing 6,000 stop start miles, because the algorithm can stretch the interval close to two years on oil that has been repeatedly cold, diluted with fuel and never boiled the moisture out of itself.

If your driving is mostly short and local, ask for the car to be switched to the fixed schedule. Most main dealers will do it, and many recommend it.

What does engine oil actually do?

Lubrication is only one of its jobs, which is why degraded oil causes such a spread of problems.

• Lubricates by keeping a film of oil between moving surfaces such as bearings, cams and piston rings.
• Cools by carrying heat away from pistons and bearings to the sump, which is a significant share of an engine's total heat rejection.
• Cleans by holding soot and combustion by products in suspension so the filter can catch them, which is why diesel oil turns black quickly and why that is normal.
• Neutralises acids produced by combustion, using an additive reserve that depletes over time.
• Seals the small gap between piston rings and bore.
• Protects against corrosion while the engine sits unused.
• Operates hydraulic components such as variable valve timing actuators, hydraulic tappets and, on some engines, timing chain tensioners.

That last point is why the wrong grade or contaminated oil causes odd faults rather than just wear. Variable valve timing units and chain tensioners rely on clean oil at the right viscosity to work correctly.

Which oil does your car need?

The specification matters far more than the brand. Any reputable oil that carries the right approval will do the job.

Viscosity is the number on the front, such as 5W-30. The first figure with the W is the cold flow rating, measured at low temperature, and lower is better for cold UK mornings. The second figure is the viscosity at operating temperature. Using an oil that is too thick does not protect better: it flows more slowly on a cold start, when most engine wear happens, and can starve narrow oil galleries and variable valve timing units.

Approvals and standards are what actually define the oil:

If your car has a diesel particulate filter, it needs a low SAPS oil (the C grades). Full SAPS oil in a DPF equipped diesel deposits ash in the filter that cannot be burnt off, permanently shortening its life. This is the most expensive oil mistake people make, and it is easy to avoid by reading the handbook.

Oil types in plain terms:

• Fully synthetic. Engineered base stock, best cold flow, best resistance to breakdown, longest service life. Required by nearly all modern engines.
• Semi synthetic. A blend, suitable for many older engines where the handbook allows it.
• Mineral. Refined crude base stock, now limited to older or classic engines.
• High mileage. Usually a synthetic blend with seal conditioners and slightly higher viscosity, aimed at worn engines that consume oil or weep from seals.

What are the signs your car needs an oil change?

Do not wait for symptoms, but know them anyway.

• The service or oil change indicator on the dashboard. On a variable schedule this is the primary instruction.
• Engine noise. A light top end rattle on start up, or a general increase in mechanical noise, often means the oil has thinned or the level has dropped.
• Low level on the dipstick. Check monthly, on level ground, with the engine off and cool or after five minutes of standing.
• Oil that is gritty between finger and thumb. Colour alone tells you little, particularly on a diesel, but grit means the filter is past its useful life.
• A burnt smell from the oil, indicating it has been overheated.
• Blue smoke from the exhaust, or oil consumption that has increased noticeably.
• Oil that looks like coffee or mayonnaise on the filler cap. On a short journey car this can be harmless condensation, but combined with coolant loss or overheating it can mean a head gasket problem.

How to check the oil properly

Park on level ground and switch the engine off. If it has been running, wait five to ten minutes for the oil to drain back into the sump, otherwise the reading is falsely low. Pull the dipstick, wipe it clean, push it fully home, then withdraw it and read the level. It should sit between the minimum and maximum marks, ideally nearer the top.

Add oil in small quantities, roughly half a litre at a time, and recheck between each addition. The gap between minimum and maximum on most cars is about a litre, so it is easy to overshoot. Cars with an electronic oil level sensor and no dipstick need the engine at operating temperature and the car level for the reading to be meaningful, and the reading updates only after the car has been standing, so check it before setting off rather than at a petrol station.

The oil change process, step by step

If you have a level surface, the right tools and somewhere to take the waste oil, this is a manageable home job. If you are not confident about supporting the car safely, it is not worth the risk.

Tools and materials

• Correct grade and quantity of oil, plus a little extra
• New oil filter, and a new sump plug sealing washer
• Oil filter wrench of the correct type, either a cup, chain or strap
• Socket or hex key for the sump plug
• Torque wrench
• Drain pan of sufficient capacity, plus a funnel
• Ramps or axle stands with wheel chocks, never a jack alone
• Gloves, rags and something to protect the drive

The procedure

• Run the engine for a few minutes so the oil is warm and flows freely. Warm, not hot: fresh drained oil at full operating temperature will scald.
• Park level, apply the handbrake, chock the wheels and raise the car safely on ramps or stands.
• Remove the oil filler cap first. This lets air in and helps the sump drain.
• Position the drain pan, remove the sump plug and let the oil drain fully. Give it ten minutes rather than two.
• Fit a new sealing washer to the sump plug and refit it to the manufacturer's torque figure. Overtightening strips alloy sumps, which is an expensive mistake.
• Remove the old filter. Spin on filters hold a quantity of oil, so position the pan first. Cartridge filters sit in a housing and need the new O rings from the kit fitted to the housing and cap.
• Smear a little clean oil on the new filter seal, then fit it hand tight plus the fraction of a turn stated on the filter body. Do not use the wrench to tighten it.
• Refill through the filler with slightly less than the stated capacity, then check the dipstick and add gradually up to the maximum mark. Overfilling is as harmful as underfilling and can push oil past seals.
• Refit the filler cap, start the engine and let it idle. The oil pressure light should go out within a few seconds. Check underneath for leaks at the plug and filter.
• Switch off, wait a few minutes for the oil to settle, then recheck the level and top up if needed.
• Reset the service indicator following the handbook procedure.
• Record the date and mileage, and keep the receipt for the oil and filter as part of the service history.

Disposing of the old oil

Used engine oil is classified as hazardous waste in the UK. Take it, and the old filter, to a household waste recycling centre with an oil bank. Pouring it down a drain, into a watercourse or onto the ground is an offence under environmental legislation and a very small quantity contaminates an enormous volume of water. Keep it in a sealed container and never mix it with other fluids, because contaminated oil cannot be recycled.

DIY or professional oil change?

The service history point matters more than people expect. A full history with dated invoices supports resale value, and on a leased or financed car it is often a condition of the agreement. If you do the work yourself, keep every receipt and log the mileage, otherwise you have effectively thrown away the paperwork value of the maintenance you paid for.

Common oil myths

• "Change it every 3,000 miles." American advice for older engines and mineral oils. Follow the handbook.
• "Thicker oil protects better." Only if the engine was designed for it. Too thick means poor cold flow and slow oil pressure build up, which is when wear happens.
• "Topping up is as good as changing." Fresh oil replenishes some additives, but it does not remove the soot, fuel dilution, acids and metal particles already in there.
• "Skipping one change now and again is fine." It is fine right up until it is not, and by then it shows as chain wear, blocked oil pick up screens and bearing damage.
• "Any 5W-30 will do." Viscosity is not the specification. The ACEA grade and manufacturer approval are what count, particularly on DPF equipped diesels.
• "Dark oil means it needs changing." In a diesel the oil goes black within a few hundred miles because it is doing its job of holding soot in suspension.

Where this fits with the rest of your car

Signature is a body shop rather than a servicing garage, so we are not the people to change your oil. We do see the results of neglected maintenance every week though, usually on cars that have been kept mechanically going but let go cosmetically, or the other way round.

If yours is due some attention on the outside, whether that is kerbed alloys, scuffed bumpers, door dents or stone chips on the bonnet, our team covers all of it through mobile SMART repairs and workshop work at Radcliffe. A free quote comes back within 24 hours, from photographs or an inspection.