ACEA, API and OEM approvals: reading the back of the bottle
Two oils marked 5W-30 can be chemically very different. The approval codes are what tell you which one your engine was designed for.
Why the grade is not enough
Viscosity describes how the oil flows. It says nothing about detergency, ash content, or how the oil behaves after 15,000 miles. Those properties are what the approval standards test, and they are the reason a bottle costs £20 or £50.
Low ash and your particulate filter
The ACEA C classes limit sulphated ash, phosphorus and sulphur, because those compounds survive combustion and collect in a diesel particulate filter. Running a full-SAPS A3/B4 oil in a DPF car will not fail immediately, but it shortens filter life measurably and the replacement is a four-figure job on some cars.
OEM codes override everything
Where a manufacturer publishes its own approval, that is the requirement. VW 504 00, BMW Longlife-04, dexos2 and Ford WSS-M2C948-B are all specific test programmes, and an oil that merely claims to be suitable has not passed them. Look for the licence number printed on the bottle.
Backwards compatibility, and where it stops
Later approvals usually supersede earlier ones within the same family: a VW 504 00 oil covers 502 00. But some are deliberately not compatible. VW 508 00 is a thinner oil for specific engines and must not be used where 504 00 is specified, even though the number is higher.
A/B for standard engines, C for cars with particulate filters, E for heavy duty.
S-codes for petrol, running alphabetically, with SP the current grade.
VW 504 00, BMW Longlife-04, dexos2, Ford WSS-M2C948-B. These override the general standards.