What Do the Numbers on the Bottle Mean?
Engine oils sold in the UK are graded under the SAE (Society of Automotive Engineers) system, written as two numbers either side of a W, for example 5W-30. Both numbers describe viscosity, how easily the oil flows, but at two different temperatures.
The first number, before the W, rates the oil's cold behaviour. Lower is thinner: a 5W oil flows more freely at low temperatures than a 10W or 15W oil, so it reaches the engine's moving parts faster on a cold start. The W stands for Winter, marking that it's a cold-weather rating, not a temperature or a season by itself.
The second number rates viscosity at 100C, close to normal running temperature, on a scale where higher means thicker. This is where the two grades actually part company: a 5W-40 measures thicker than a 5W-30 once the engine is hot, even though both flow almost identically from cold, since they share the same 5W rating there.
5W-30 vs 5W-40 in Practice
That one difference, how thick the oil runs once hot, is usually what steers a manufacturer's choice of grade.
5W-30 is the more common specification on modern petrol engines and smaller diesels. A thinner oil at operating temperature means less internal drag for the moving parts and pump to overcome, which typically helps cold-start economy and, on some engines, contributes to the headline fuel figures. It has become the default on many newer engines built to tolerances that don't call for a thicker protective film.
5W-40 tends to appear where an engine runs hotter or under heavier sustained load: some turbocharged and performance engines, some older engine designs with wider bearing clearances, and higher-mileage engines that can benefit from the extra film thickness as wear opens up tolerances. The thicker hot-viscosity film is there to keep metal surfaces apart when temperature and load are working hardest against the oil.
Neither grade is universally the better choice. Fitting 5W-40 to an engine designed around 5W-30 doesn't automatically add protection, and fitting 5W-30 to an engine calling for 5W-40 isn't automatically the more efficient option. Each is a trade-off tuned to a specific engine design.
The Handbook Spec Wins Every Time
Viscosity really is only half the story, and it gets all the attention because it's the part printed largest on the bottle.
The other half is the approval code, something like an ACEA rating (A3/B4 is a common petrol/diesel spec) or a manufacturer-specific approval in a VW 504.00 style. These codes cover what a viscosity number alone can't: the detergent and additive package, how the oil holds up over a long drain interval, and compatibility with specific engine hardware and emissions equipment. Two oils can carry the identical 5W-30 grade and still differ in which approval codes they hold, so matching the viscosity number without checking the approval code is only doing half the job.
This matters most on diesels fitted with a diesel particulate filter (DPF). These typically need a low-SAPS oil (low Sulphated Ash, Phosphorus and Sulphur content), usually shown as a C-grade approval such as ACEA C2 or C3, because a high-ash oil can contribute to faster DPF clogging over time. An oil can be the correct viscosity and still be the wrong choice for a DPF-equipped car if it isn't the right low-SAPS spec, so this is a case-by-case check against the handbook rather than a rule to generalise from.
The reliable route stays the same regardless of grade: whatever the vehicle handbook or the oil filler cap states, in both viscosity and approval code, is the one to buy. Once that spec is confirmed, our oil service cost calculator turns the capacity figure and current prices into a full DIY-versus-garage total.
Can You Mix or Switch Grades?
Day to day, sticking to one consistent spec matched to the handbook beats swapping grades between changes for no particular reason.
That said, if a car is genuinely low on oil and only the other grade is available, topping up with it is generally the better short-term choice over continuing to run low, since a low oil level risks more immediate harm than briefly mixing two closely related grades. It isn't something to make a habit of, and a full drain and refill with the correct spec at the next opportunity is the tidy way to put it right.
Quick Answers
Is a thicker oil always better for an older engine? Sometimes, but not automatically. Higher-mileage engines with wider bearing clearances can benefit from the extra film a 5W-40 provides, but the manufacturer's specification for that engine is still the starting point, not a rule that older always means thicker.
Does running 5W-40 instead of 5W-30 hurt fuel economy? Typically only marginally, since the extra hot-viscosity drag is a small fraction of an engine's overall friction, though on an engine specifically tuned around a thinner oil the difference can be more noticeable than on one where either grade is approved.
What does the W actually stand for? Winter. It marks the cold-weather rating the first number is based on, not a temperature value or a season by itself.