Engine Oil Grades Explained: 5W-30, 5W-40 And Manufacturer Specs
Car Parts Centre · 1 August 2026
Viscosity numbers, ACEA and OEM approvals, and why the spec on the bottle matters more than the price.
Buying oil looks simple until you see six bottles that all say 5W-30. The viscosity grade is only half the story — the approvals printed underneath decide whether the oil protects your engine and keeps your warranty and DPF intact.
What the numbers mean
In 5W-30, the 5W describes cold-start flow (W for winter) and the 30 describes thickness at operating temperature. A lower first number flows faster on a frosty Yorkshire morning, when most engine wear happens. The second number must match what the manufacturer specifies — thicker is not better.
Approvals matter more than grade
Two 5W-30 oils can be completely different products. A low-SAPS C3 oil protects diesel particulate filters and catalysts; a full-SAPS A3/B4 oil does not, and will clog a DPF over time.
- ACEA C1/C2/C3/C5 — low or mid SAPS, for DPF-equipped cars.
- ACEA A3/B4 — higher SAPS, older petrol and diesel engines.
- OEM approvals: VW 504 00/507 00, BMW LL-04, MB 229.51, Ford WSS-M2C913-D, Dexos.
- If the handbook names an approval, the bottle must list it.
How much oil do I need?
Most 1.0–2.0 litre cars take between 3.5 and 5.5 litres including the filter. A 5-litre container covers the majority of services with a little left for topping up; check the level with the dipstick or the dashboard sensor after the engine has stood for a few minutes.
Change the filter every time
An oil change without a new filter leaves the old contaminated element and its trapped debris in circulation. Fit a new sump plug washer too — most are single-use crush washers and cost pennies compared with a stripped sump.
Frequently asked questions
Can I mix 5W-30 and 5W-40?
In an emergency top-up, yes. As a long-term choice, no — use the grade and approval in your handbook.
Is fully synthetic worth it?
For modern turbocharged and DPF-equipped engines it is usually required, not optional.
How often should oil be changed?
Every 10,000–12,000 miles or annually for most cars; short-journey and towing use benefits from more frequent changes.