The workshop consumables checklist: what to keep on the shelf
Oils
Which engine oil grade? What 5W-30 means and why the spec matters more
Most people walk up to the counter knowing one thing about the oil their car takes: a pair of numbers like 5W-30. That is a good start, but it is only the viscosity, and two 5W-30s can be very different oils. Here is how to read the whole label, what actually decides whether an oil suits your engine, and when it is worth paying for a full synthetic.
What 5W-30 actually means
The grade is an SAE viscosity rating, and it describes how thick the oil is at two temperatures. The first number, with the W for winter, is how the oil flows when cold: the lower it is, the more easily the oil reaches the top of the engine on a cold start, which is when most wear happens. The second number is the oil's thickness at normal operating temperature, around 100 degrees Celsius. So a 5W-30 flows like a thin oil on a cold morning and protects like a 30-weight once the engine is warm. A 10W-40 is thicker cold and thicker hot. Neither is better; the engine was designed around one of them, with bearing clearances and oil galleries sized to suit.
Viscosity is the grade, the spec is the quality
The part of the label people skip is the performance specification, and it matters more than the grade. The American Petroleum Institute (API) rates petrol oils with an S category (SN, SN Plus, SP) and diesel oils with a C category (CI-4, CJ-4, CK-4); each new letter supersedes the last. ACEA, the European body, uses A/B categories for petrol and light diesel and C categories for engines with particulate filters and catalysts, where a low-SAPS oil (low sulphated ash, phosphorus and sulphur) is needed to stop the filter clogging. ILSAC GF-6 covers fuel economy for modern petrol engines. On top of those, many manufacturers publish their own approvals, and a European car in particular will often list one of those as a requirement rather than a suggestion.
Short version: the grade has to match, and the specification has to meet or exceed what the handbook calls for. An oil that gets the grade right and the spec wrong can still damage a modern engine.
Check the handbook first
Every one of these answers is in the owner's handbook, usually in the service or specifications section: the grade, the specification, often a range of acceptable grades for different climates, and the oil capacity with the filter. If the handbook is long gone, the vehicle finder on this site will narrow the range to oils that match your make and model, and the team can check the specification against the vehicle for you. Do not rely on what the last oil change used; the previous owner may have guessed.
Petrol and diesel are not interchangeable
Diesel oils carry stronger detergents and dispersants because a diesel makes more soot and more acid in the combustion process, and the oil has to hold that soot in suspension between changes. Petrol oils are blended for a spark-ignition engine and for its catalytic converter. Some oils carry both an S and a C rating and are genuinely dual-use, but a heavy-duty truck oil in a small petrol car or a petrol oil in a common-rail diesel is the wrong tool. If the vehicle has a diesel particulate filter, the oil must be a low-SAPS grade, whatever else it says on the front.
When full synthetic is worth it
A full synthetic base oil is more uniform than a mineral one, so it resists breaking down under heat, flows better when cold and keeps its viscosity for longer. It costs more, and in some situations the extra is well spent:
- Turbocharged and direct-injection engines, which run the oil hotter and are prone to low speed pre-ignition; an SN Plus or SP rated synthetic is formulated against it
- Towing, heavy loads and hot summers, where thermal stability is what stops the oil thinning out
- Long drain intervals, where the oil has to stay in grade for 10,000 to 15,000 km
- Any engine whose handbook simply specifies a synthetic or a manufacturer approval that only synthetics meet
When mineral or semi-synthetic is fine
An older engine with simpler needs, serviced at shorter intervals and driven gently, does not need the top shelf. Many older engines are happier on a mineral or semi-synthetic of a slightly heavier grade, which seals better on worn rings and reduces consumption. The rule is the same: meet the specification the engine was built for, and if the handbook offers a range of grades, pick the heavier end for hot climates and high mileage.
Bring the details, leave with the right oil
If you are not sure, bring the make, model, year and engine, and whether it is petrol or diesel. With those, the counter can match the grade and the specification in a minute. Buy the filter at the same time, because an oil change without a new filter is only half a service.
