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Workshop guide

Engine oil specifications explained

A quick reference for UK workshops and technicians: what the numbers on an oil bottle actually mean, how ACEA and API specifications differ, and how to match the right oil to a passenger car.

Viscosity grades (SAE)

Viscosity describes how thick the oil is at temperature. A grade like 5W-30 has two parts: the number before the W is the cold-start (winter) viscosity, and the number after is the viscosity at operating temperature (100°C). Lower cold numbers flow faster on a cold morning; higher hot numbers give a thicker film at temperature.

  • 0W-20 / 0W-30 — modern low-friction engines, hybrids, many post-2015 petrols specifying low HTHS oils.
  • 5W-30 — the most common UK grade across VAG, Ford, PSA, Vauxhall petrols and diesels with DPFs (usually a low-SAPS variant).
  • 5W-40 — high-mileage or performance petrols and older TDI/HDi diesels.
  • 10W-40 — older vehicles, semi-synthetic service fills.

ACEA sequences

ACEA is the European manufacturer standard and matters more than API for UK cars. The letter identifies the engine family and the number is the performance level:

  • A/B — petrol and light-duty diesel without particulate filter constraints (older cars).
  • C1 / C2 / C3 / C4 / C5 — low- and mid-SAPS oils for engines with DPFs or GPFs. Most modern UK service fills need a C-sequence oil. Using a non-C oil can block the DPF.
  • E — heavy-duty diesel; not used on passenger cars.

API service categories

API is the American standard. On petrol cars the current categories are SP (with LSPI protection) and SN Plus; on diesels CK-4 is current. UK passenger-car service is normally driven by the manufacturer approval and the ACEA sequence, with API as an additional line on the bottle.

Manufacturer approvals

Manufacturer approvals are the specifications the vehicle actually calls for in the handbook — for example VW 504.00 / 507.00, MB 229.51, BMW LL-04, Ford WSS-M2C913-D, or PSA B71 2312. An oil can be 5W-30 and still be wrong if it doesn't carry the required approval. Always match the approval printed in the service literature, not just the viscosity.

How to pick the right oil

  1. Identify the vehicle by VIN or registration.
  2. Read the manufacturer approval from the service data (e.g. MB 229.51).
  3. Confirm the ACEA sequence (usually C2 or C3 on modern diesels).
  4. Match the SAE viscosity to the manufacturer's temperature chart.
  5. Check the sump capacity so the refill quantity is right.

OilSpec automates steps 1–5 from a photo of the plate or VIN. Create a workshop account to try a live lookup.