API Engine Oil Categories Explained
The SAE grade is thickness. The API category is the performance claim. S is petrol, C is diesel. For a new petrol licence the current category is API SQ, paired with ILSAC GF-7 (Starburst for most grades, Shield for 0W-16). API SP and ILSAC GF-6 are still on shelves; they are no longer the mark a 2026 development should aim at. Diesel stays CK-4 or FA-4.
Two families, and a level within each
An API category is always two letters. The first letter names the engine family; the second (and any suffix) names the performance level, which rises alphabetically as the standard is revised:
- S = Service / Spark-ignition — petrol car and motorcycle engines. Think “S for Spark.”
- C = Commercial / Compression-ignition — diesel trucks, buses, off-highway and generators. Think “C for Compression.”
So API SQ is the current petrol level for a new licence; API SP is the 2020 level still on shelves; API CK-4 is a current diesel level. A higher second letter is a newer standard — SQ above SP above SN. The category sits on the additive package: it is the DI chemistry, not the base oil alone, that earns the letters.
API pairs petrol fuel-economy oils with ILSAC GF-7. Indian retail also references IS 13656. The 0W-16 case is GF-7B, not the Starburst.
The S series — petrol engine oils
| Category | Since | Status | What it added |
|---|---|---|---|
| API SP | 2020 | Current | LSPI protection, timing-chain wear, cleaner turbos |
| API SN Plus | 2018 | Current | First LSPI safeguard (interim to SP) |
| API SN | 2010 | Current | Better sludge, seal and turbo protection |
| API SM / SL | 2004 / 2001 | Usable | Older cars; superseded by SN/SP |
| API SA–SJ | legacy | Obsolete | Not for modern engines |
The big recent driver is LSPI — Low-Speed Pre-Ignition — a damaging knock in small turbocharged direct-injection petrol engines. API SN Plus and then SP re-balanced detergent chemistry (notably the calcium/magnesium detergent mix) to suppress it. SP is backward compatible, so it can replace SN, SM and older oils.
The C series — diesel engine oils
| Category | Since | Status | Notes |
|---|---|---|---|
| API CK-4 | 2016 | Current | Full HTHS (≥ 3.5); backward compatible with CJ-4 |
| API FA-4 | 2016 | Current | Low HTHS (2.9–3.2) for fuel economy; not backward compatible |
| API CJ-4 | 2006 | Usable | Low-SAPS for DPF-equipped engines |
| API CI-4 / CH-4 | 2002 / 1998 | Usable | Older EGR / pre-DPF diesels |
CK-4 and FA-4 arrived together but are not interchangeable. CK-4 keeps the traditional high HTHS and drops into older engines; FA-4 runs a lower HTHS to save fuel and must only be used where the engine maker specifies it. This is the one place where a “newer” diesel oil is not automatically a safe upgrade — see our 15W-40 CK-4 formulation.
The API donut, the starburst and ILSAC
The API “donut”
The round Service Symbol on the back of the bottle: the top arc shows the API category (e.g. SP), the centre shows the SAE grade (e.g. 5W-30), and the lower arc shows “Resource Conserving” if it qualifies.
The starburst
The ILSAC mark for petrol grades other than SAE 0W-16. From 31 March 2026 that mark is GF-7A. GF-6A is no longer eligible for a new Starburst.
The shield
ILSAC GF-7B, and only SAE 0W-16. A 0W-20 cannot wear it. See the 0W-16 formula note.
Dual-rated oils
Some oils carry both an S and a C rating (e.g. API SP / CK-4 style claims via mixed fleet oils), but the categories are optimised differently. For a mixed petrol/diesel fleet we formulate deliberately, not by stacking claims.
Formulating to an API category
Meeting an API category is an additive-and-testing problem. Lubechem selects the DI package and base-oil group for manufacturers — SP, CK-4 or FA-4 — then proves the result against the ASTM tests behind the category. You own the IP.
API oil category FAQ
What does API SQ mean on engine oil?
API SQ is the petrol category licensed from 31 March 2025. It sits above SP. New passenger-car work should name SQ and the matching ILSAC GF-7 mark. SP oils already in the market were not relabelled.
What does API SP mean on engine oil?
API SP is the 2020 petrol category. It added low-speed pre-ignition and timing-chain protection over SN and remains backward compatible with older S grades. It is no longer the newest licence.
Is API SP backward compatible with SN?
Yes. The S-series is designed so each new category covers the ones before it. An API SP oil meets and exceeds API SN, SN Plus, SM and SL requirements, so it can safely replace them in petrol engines that called for the older grade.
What is the difference between CK-4 and FA-4?
Both are 2016 diesel categories. CK-4 keeps the traditional high high-shear viscosity (HTHS ≥ 3.5 mPa·s) and is backward compatible with CJ-4 engines. FA-4 uses a lower HTHS (2.9–3.2 mPa·s) for fuel economy and is NOT backward compatible — it must only be used where the engine manufacturer specifically approves it.
What is the difference between the S and C API categories?
S categories are for petrol (spark-ignition) engines; C categories are for diesel (compression-ignition) engines. They test against different engine hardware and deposit/soot challenges, so a petrol SP oil and a diesel CK-4 oil are optimised differently even at the same SAE grade.
How is API different from ILSAC and BIS?
API sets the engine-oil categories (SQ, SP, CK-4). ILSAC adds the fuel-economy petrol mark: GF-7A Starburst, or GF-7B Shield for 0W-16 only. BIS IS 13656 is the Indian standard used for local certification. A formula is built against the marks you intend to print.
Lubechem formulates oils to API SP, CK-4 and FA-4 for manufacturers
If you want the viscosity language first, start with SAE grades or the 5W-30 vs 5W-40 vs 10W-40 comparison.
SAE viscosity grades explained →