What does 15W-40 mean on an oil label?

It's the SAE viscosity classification. The '15W' indicates COLD-weather behavior (winter) — the lower the number, the better it flows in the cold. Example: a 5W flows better cold than a 20W. The '40' indicates HOT viscosity at 100°C — the higher the number, the thicker it is in operation. Example: a 40 is thicker hot than a 30. A 15W-40 oil is MULTIGRADE — it behaves like a 15W when cold and like a 40 when hot. It's the most common grade in heavy-duty diesel engines in temperate climates.

What's the difference between CI-4, CJ-4, and CK-4?

These are API categories for diesel engines, read 'CI-4,' 'CJ-4,' 'CK-4' with increasing letters. A later letter means more modern = better properties. CI-4 (2002): pre-DPF diesel engines. High zinc/phosphorus. CJ-4 (2007): diesel engines with DPF and SCR. Reduced zinc so it doesn't poison catalysts. CK-4 (2017): an improvement over CJ-4, optimized for extended drain. FA-4 (2017): lower viscosity (10W-30 or 15W-30) for efficiency. On a modern engine (post-2007), ALWAYS use CJ-4 or CK-4. Using CI-4 (an older category) will clog the DPF within a few months.

What is ISO 46 and when is it used?

ISO 46 is a viscosity classification for HYDRAULIC oils (not engine oil). It indicates the oil has a viscosity of 46 centistokes at 40°C. It's the STANDARD viscosity for heavy construction equipment (excavators, loaders, bulldozers) in temperate climates. Other common ISO grades: ISO 32 (cold climates), ISO 68 (hot climates or heavy loads), ISO 100 (very demanding applications). Hydraulic oils use the ISO classification, not SAE — they're different systems.

Can I use engine oil in a hydraulic system?

NO, generally that's not a good idea. Even though they look like similar liquids, they have VERY DIFFERENT additives. Engine oil: strong detergents to deal with soot. Hydraulic oil: specific anti-wear additives + reinforced anti-foaming agents + low sludge formation. If you use engine oil in a hydraulic system: the engine detergents can emulsify any water and drag contaminants into the filter (saturating it), can foam during pumping, and won't give the specific protection the hydraulic system needs. In a one-off emergency it works; as a regular practice it greatly shortens the system's service life.

What's the difference between SN, SP for cars, and CJ-4, CK-4 for diesel?

API categories split into two groups by fuel type: 'S' (Service / Spark ignition = gasoline) and 'C' (Commercial / Compression ignition = diesel). Gasoline cars use S categories: SN, SP, SQ (the most modern). DIESEL engines use C categories: CJ-4, CK-4, FA-4. The second letter indicates the generation (later letter = more modern). Never use an S-category oil in a diesel engine or vice versa — the additives are optimized for each combustion type.

Oil Classifications: API, SAE, and ISO Explained

When you buy oil, you see codes like 15W-40 CK-4 API SN Plus. Each part of that label means something specific. This article teaches you how to read the three main classifications: API (oil quality), SAE (viscosity), and ISO (viscosity for hydraulic oils).

API (American Petroleum Institute)#

API defines quality levels and compatibility of oil with different engines.

API oils split into two groups by engine type:

S Group: gasoline engines#

S = Service or Spark ignition.

Codes: SN, SP, SQ (the most modern).

SN (2010): gasoline engines from 2011+.
SP (2020): gasoline engines from 2020+ with LSPI (Low-Speed Pre-Ignition prevention) technology.
SQ (2024+): latest generation.

The letters progress through the alphabet — each new category is backward-compatible with earlier ones.

C Group: diesel engines#

C = Commercial or Compression ignition.

Codes: CJ-4, CK-4, FA-4.

CI-4 (2002): pre-2007 diesel engines with EGR but no DPF. High ZDDP (1,200 ppm).
CJ-4 (2007): engines with DPF and SCR. ZDDP reduced to 1,200 ppm max. Optimized detergent-dispersants.
CK-4 (2017): an improvement over CJ-4. Optimized for extended drain. Better shear stability.
FA-4 (2017): lower viscosities (10W-30, 15W-30) for fuel efficiency. Compatible with specific 2017+ engines.

How to choose the right API category#

General rule: use the most modern category compatible with your engine.

2007-2016 diesel engine with DPF/SCR: use CJ-4 or CK-4.
2017+ diesel engine: use CK-4 or FA-4 (if approved by the manual).
Pre-2007 diesel engine (no DPF): can use CI-4 or a more modern category (also works).

NEVER use an older category in a more modern engine. Example: using CI-4 in an engine with a DPF will clog the DPF within a few months.

SAE (Society of Automotive Engineers)#

SAE defines the oil’s viscosity — how well it flows at different temperatures.

Monograde oils#

A single number. Examples:

SAE 30: medium viscosity, typical in moderate climates.
SAE 40: thicker, engines with more blow-by.
SAE 50: very thick, older worn engines.

Monogrades are used less and less — they don’t perform well across variable climates.

Multigrade oils#

Two numbers separated by “W” (winter). The most common is 15W-40.

Format: XW-Y

X: cold (winter) viscosity.
Y: hot viscosity (at 100°C).

The W number (cold)#

Indicates how well the oil flows in the cold — specifically at sub-zero temperatures.

W Grade	Flows down to
0W	-40°C
5W	-35°C
10W	-30°C
15W	-25°C
20W	-15°C
25W	-10°C

Lower W number = better cold-weather performance.

The final number (hot)#

Indicates viscosity at 100°C in centistokes.

Grade	Viscosity at 100°C (cSt)	Typical use
20	5.6-9.3	Modern light-duty cars
30	9.3-12.5	Cars and medium diesel engines
40	12.5-16.3	Heavy diesel engines
50	16.3-21.9	Very old engines or high load
60	21.9-26.1	Extreme applications

Higher number = thicker when hot.

Common examples in diesel engines#

15W-40: the most common in temperate climates.
10W-40: better starts in moderate cold.
5W-40 synthetic: cold climates.
10W-30 synthetic: modern fuel-efficient engines.
15W-30: modern extended-drain (FA-4).

Choosing the right viscosity#

Depends on:

The coldest ambient temperature the engine will experience.
Normal operating temperature.
Engine type and condition.
The manufacturer’s specification.

Basic rule:

Cold climate (-25°C to 20°C): synthetic 5W-40 or 10W-40.
Temperate climate (0°C to 35°C): mineral or semi-synthetic 15W-40.
Hot climate (20°C to 45°C): 15W-40 or 20W-50.

Engine with heavy wear (high blow-by): use a higher viscosity (15W-50 or 20W-50) for better sealing.

Very new engine with tight tolerances: follow the manufacturer’s exact specification — don’t experiment.

ISO (International Organization for Standardization)#

ISO classifies HYDRAULIC oils by viscosity. It’s not used for engine oils.

Common ISO grades#

Grade	Viscosity at 40°C (cSt)	Typical use
ISO 22	22	Very cold climates
ISO 32	32	Cold climates, low load
ISO 46	46	STANDARD in heavy equipment
ISO 68	68	Hot climate or heavy loads
ISO 100	100	Demanding applications
ISO 150	150	Very high loads, low RPM

The most common in heavy construction equipment (excavators, loaders, bulldozers): ISO 46.

ISO H, HM, HV, HVI#

Along with the numeric grade, ISO specifications add letters:

HL: mineral hydraulic oil with oxidation and corrosion inhibitors. General use.
HM: mineral oil with anti-wear additives. Heavy-duty equipment.
HV: mineral oil with viscosity index improvers. Variable climates.
HVI: high viscosity index — very stable against temperature changes.

Typical example on an excavator: ISO 46 HM or ISO 46 HV hydraulic oil.

How to read a complete label#

Typical example: a commercial diesel engine oil.

CATERPILLAR DEO 15W-40 API CJ-4/CI-4 ACEA E9/E7

Deciphering it:

CATERPILLAR DEO: brand and product.
15W-40: SAE viscosity (15W cold, 40 hot).
API CJ-4/CI-4: meets API category CJ-4 (engines with a DPF) and CI-4 (pre-2007 engines). Backward-compatible.
ACEA E9/E7: meets European ACEA standards for heavy trucks.

Another example, for hydraulic oil:

MOBIL DTE 25 ISO 46 HM

MOBIL DTE 25: brand and product.
ISO 46: viscosity of 46 cSt at 40°C.
HM: mineral oil with anti-wear additives.

Other classifications worth knowing#

ACEA (Europe)#

A European system similar to API but with different logic:

A/B: gasoline and light-duty diesel engines.
C: engines with aftertreatment (DPF, SCR).
E: heavy diesel engines.

Examples: ACEA E7 (equivalent to CI-4/CJ-4), ACEA E9 (equivalent to CJ-4/CK-4).

JASO (Japan)#

A Japanese system, mainly for motorcycles and specific applications.

JASO MA: 4-stroke motorcycles with wet clutches.
JASO MB: 4-stroke motorcycles with dry clutches.
JASO DH-1/DH-2: Japanese diesel engines.

Manufacturer specifications#

Every major manufacturer has its own specifications:

Caterpillar: ECF-3, ECF-2 (for its engines).
Cummins: CES 20081, CES 20086.
Volvo: VDS-3, VDS-4, VDS-4.5.
Mercedes: MB 228.51, MB 228.52.
Deutz: DQC III, DQC IV.

A good commercial oil usually meets several of these specifications at once, all listed on its label.

In summary#

The three key classification systems:

API: quality and engine compatibility. S (gasoline) and C (diesel) groups. Modern diesel engines require CJ-4 or CK-4.

SAE: oil viscosity. Multigrades like 15W-40 are the standard in temperate-climate diesel engines. The W number indicates cold performance; the final number indicates hot performance.

ISO: viscosity for hydraulic oils. ISO 46 is the standard in heavy equipment. It’s NOT used for engine oils.

Golden rule: always check the manufacturer’s manual to know the exact specification your engine requires. Never use older categories than what the engine requires. When oils of different specifications get mixed, the protection of the more modern one is lost.

This article was drafted with AI assistance and reviewed by our editorial team before publishing.