Oil is chosen on price and brand far more often than on the specification the manufacturer actually requires.
Viscosity grades
Two numbers describing flow behaviour cold and hot.
Which must match what the engine was designed for.
Performance standards
Industry and manufacturer approvals.
Which certify the additive package rather than the viscosity.
Why it matters more now
Modern engines with tight tolerances, turbochargers and particulate filters.
Which are sensitive to oil specification in ways older engines were not.
Change intervals
Extended intervals depend on the correct oil being used.
Reading a viscosity grade
The first number with a W describes cold flow and the second describes viscosity at operating temperature.
Which together determine how the oil behaves across the range the engine sees.
A thicker oil than specified does not provide more protection in a modern engine and can restrict flow to critical parts.
Synthetic and mineral
Base oil types with different stability and cost.
Which is a separate question from the specification.
Most modern engines require fully synthetic to meet the approval.
Manufacturer approvals
Specific codes listed in the handbook.
Which are more binding than a general industry standard.
Using an unapproved oil can affect warranty cover.
Particulate filters
Low ash oils required to avoid clogging.
Which is why some diesels specify a particular formulation.
Where to check
The handbook, the oil cap and the manufacturer's online lookup.
Why the wrong oil causes damage
Modern engines rely on oil for hydraulic actuation as well as lubrication.
Which includes variable valve timing and timing chain tensioners.
Incorrect viscosity can prevent these systems operating as designed.
Oil consumption
Some engines consume oil normally between services.
Which makes checking the level a genuine maintenance task rather than a formality.
Running low is a common cause of expensive failure on modern turbocharged engines.
Service intervals
Fixed and variable schedules.
Which for short-journey urban driving are frequently too long in practice.
Many technicians advise halving extended intervals for that use pattern.
Topping up between grades
Mixing oils in an emergency is better than running low.
Which is a practical point worth knowing.
Where to check
The handbook lists the required specification, and manufacturer lookup tools confirm it by registration.
How to find the right specification
The handbook is the primary source, followed by the manufacturer's registration lookup.
Which removes the guesswork entirely and takes about a minute.
Parts retailers offer lookup tools that are usually accurate and occasionally suggest a close alternative rather than the exact approval.
Diesel and petrol differences
Different additive requirements and filter compatibility.
Which makes the approval codes more important than the viscosity on diesels with particulate filters.
Long-life servicing
Extended intervals requiring specific oils and driving conditions.
Which are calculated by the vehicle from actual use in many cases.
Predominantly short journeys frequently trigger earlier service demands, which is the system working correctly.
Checking the level
Cold, on level ground, using dipstick or electronic display.
Which should be part of a monthly routine alongside tyre pressures.
Cost against consequence
The price difference between correct and incorrect oil is small.
Which makes this one of the least sensible places in vehicle maintenance to economise.
Common misconceptions
That thicker oil protects better, that synthetic causes leaks in older engines, and that any oil of the right viscosity will do.
Which are all widely repeated and none accurate for modern engines.
The approval code exists precisely because viscosity alone does not describe what the engine needs.
Older engines
Different requirements and frequently a different viscosity grade.
Which is why classic and older vehicles use oils that would be wrong in a current engine.
High-zinc formulations exist for engines with flat tappet camshafts.
Motorcycle oils
Wet clutches sharing engine oil.
Which requires friction modifiers to be absent.
Using a car oil in such an engine causes clutch slip.
Disposal
Used oil is a regulated waste.
Which local recycling facilities accept free in most places.
The summary
Look up the approval code, buy an oil that carries it, check the level monthly, and change it on the schedule.
A general note on sources
Figures in this area come from manufacturer publications, regulator test programmes, insurance claims data and independent consumer testing, and those four sources do not always agree.
Where they conflict, the independent testing and the real-world claims data are usually the more reliable guide, because manufacturer figures are produced under conditions chosen by the manufacturer.
Anything specific to your own vehicle should be checked against its handbook and against a qualified technician familiar with the model, since specifications differ between markets and between production years in ways that general articles cannot capture.
Where to go for model-specific detail
Owner forums and marque specialists know more about any individual model than any general article can, including which components fail and roughly what the repair costs.
An hour reading before a purchase or a repair decision is consistently one of the better uses of time available to a vehicle owner.