SAE Viscosity Grades Explained
A grade like 5W-30 is two numbers and a promise: the oil will crank and pump cold enough for the “5W” winter rating, and hold a defined viscosity when hot enough for the “30” rating. The Society of Automotive Engineers standard SAE J300 sets the exact test limits behind each number — measured in cranking viscosity, pumping viscosity, kinematic viscosity and high-shear viscosity. Here is what every part of a grade means, and how to read one.
A grade is a cold promise and a hot promise
Engine oil has to be thin enough to pump at a freezing cold start, yet thick enough to hold an oil film at full operating temperature. Those are opposing demands, and SAE J300 grades them separately:
- The number with a “W” (0W, 5W, 10W, 15W, 20W, 25W) is the winter rating — how the oil behaves cold. Lower W = flows at a lower temperature.
- The number without a W (20, 30, 40, 50, 60) is the hot rating — the viscosity at 100 °C and under high shear. Higher = thicker film when hot.
An oil that meets both a winter and a hot rating — like 5W-30 — is a multigrade. A single-rating oil (just SAE 30) is a monograde. Multigrades exist because a viscosity index improver lets one oil satisfy a cold limit and a hot limit — the core trick of modern engine oil.
“W” stands for Winter, not weight. A 5W-30 is not “5-weight” oil; it is an oil that passes the 5W cold tests and the SAE 30 hot tests.
Winter grades — the cold-start limits
Each W grade must pass two low-temperature tests, at a temperature that gets colder as the grade number drops. Cranking viscosity (CCS, ASTM D5293) predicts whether the starter can turn the engine; pumping viscosity (MRV, ASTM D4684) predicts whether the pump can move oil before a bearing runs dry.
| W grade | Max cranking (CCS) | Max pumping (MRV) | Min KV @ 100 °C |
|---|---|---|---|
| 0W | 6200 mPa·s @ −35 °C | 60,000 @ −40 °C | 3.8 mm²/s |
| 5W | 6600 mPa·s @ −30 °C | 60,000 @ −35 °C | 3.8 mm²/s |
| 10W | 7000 mPa·s @ −25 °C | 60,000 @ −30 °C | 4.1 mm²/s |
| 15W | 7000 mPa·s @ −20 °C | 60,000 @ −25 °C | 5.6 mm²/s |
| 20W | 9500 mPa·s @ −15 °C | 60,000 @ −20 °C | 5.6 mm²/s |
| 25W | 13,000 mPa·s @ −10 °C | 60,000 @ −15 °C | 9.3 mm²/s |
A lower W grade needs a better base oil (Group III or IV) and a pour point depressant — which is why 0W and 5W oils cost more than 15W or 20W. In most of India a 15W or 20W winter rating is enough; 0W/5W is for genuine cold starts or fuel-economy claims.
High-temperature grades — viscosity when it counts
The second number is set by two hot measurements: kinematic viscosity at 100 °C (KV100, ASTM D445) and high-temperature high-shear viscosity at 150 °C (HTHS, ASTM D4683/D4741). KV100 puts the oil in a grade band; HTHS is the number that actually protects a loaded bearing at speed — and the one modern specs care most about.
| Grade | KV @ 100 °C (mm²/s) | Min HTHS @ 150 °C | Typical use |
|---|---|---|---|
| 20 | 6.9 – <9.3 | 2.6 mPa·s | Modern petrol, fuel economy |
| 30 | 9.3 – <12.5 | 2.9 mPa·s | Passenger cars, most modern PCMO |
| 40 | 12.5 – <16.3 | 3.5 / 3.7 mPa·s | Diesel, high-load, hot climates |
| 50 | 16.3 – <21.9 | 3.7 mPa·s | Older / air-cooled, heavy duty |
| 60 | 21.9 – <26.1 | 3.7 mPa·s | Racing, very high temperature |
For grade 40 the HTHS minimum is 3.5 mPa·s in 0W-40/5W-40/10W-40 and 3.7 mPa·s in 15W-40/20W-40/25W-40. SAE has also added thinner fuel-economy grades — 8, 12 and 16 — with HTHS down to 1.7 mPa·s for the latest low-viscosity engines.
How to read 5W-30 and 15W-40
5W-30
Cranks and pumps at −30 °C / −35 °C (the 5W limits), and sits in the 30 band hot (KV100 9.3–12.5, HTHS ≥ 2.9). A wide-temperature passenger-car oil — needs a Group III or PAO base and a shear-stable VII. This is the mainstream modern petrol grade.
15W-40
Cranks at −20 °C (15W) and sits in the thicker 40 band hot (KV100 12.5–16.3, HTHS ≥ 3.7). The workhorse Indian diesel grade — a robust film for high-load CI engines, achievable on a Group II base. See our 15W-40 formulation.
0W-20
Cranks/pumps at −35 °C / −40 °C and runs thin hot (KV100 6.9–9.3, HTHS ≥ 2.6). A fuel-economy grade for tight modern engines — demands a top base oil and precise additive balance. See our 0W-20 formulation.
SAE 40 (mono)
A monograde — only the hot rating. No W means no low-temperature guarantee, so it is used where cold starts do not matter: some stationary engines, gearboxes and older equipment.
The gap between the two numbers is a measure of how “multigrade” the oil is. A 0W-40 spans a huge range and leans hard on additive chemistry; a 20W-40 spans little and is easy to build. Widening that span cheaply, without the VII shearing down in service, is a core formulation skill.
Formulating to a target SAE grade
Tell us the grade and the spec — say a 5W-30 API SP, or a 15W-40 CK-4 — and we build the recipe backwards from the J300 limits: the base-oil blend to hit KV100 and cold flow, the VII and PPD to bridge cold-to-hot, and the DI package for wear and cleanliness — then prove every J300 and API number in the lab. You own the formula and the IP.
Related guides
SAE viscosity grade FAQ
What does the W in 5W-30 stand for?
The W stands for Winter — it is the cold-temperature rating, not the oil’s weight. A 5W oil must pass cranking (CCS) and pumping (MRV) viscosity limits at low temperature; the lower the W number, the colder the oil can still start and circulate.
What is the difference between 5W-30 and 5W-40?
Both share the same 5W cold rating, so they crank and pump identically in the cold. The difference is hot: 5W-40 is thicker at 100 °C (KV100 12.5–16.3, HTHS ≥ 3.5) than 5W-30 (KV100 9.3–12.5, HTHS ≥ 2.9). 5W-40 gives a stronger high-temperature film; 5W-30 gives slightly better fuel economy.
What does HTHS mean and why does it matter?
HTHS is High-Temperature High-Shear viscosity, measured at 150 °C under high shear (ASTM D4683). It represents the real viscosity in a loaded, fast-moving bearing — closer to service conditions than KV100. A higher HTHS protects better; a lower HTHS saves fuel. Modern specs set HTHS minimums per grade.
Is a monograde like SAE 40 still useful?
Yes, where cold starts are not a concern — some stationary engines, older equipment, gear applications and run-in oils. A monograde has only a hot rating and no low-temperature guarantee, but it needs no viscosity-index improver, so it is simple and shear-stable.
Which SAE grade should I use in India?
For most Indian passenger cars, a 5W-30 or 10W-30 matching the OEM spec is ideal; for diesel and commercial engines, 15W-40 is the workhorse grade. In genuinely cold regions or for fuel-economy claims, 0W/5W grades are worth the higher base-oil cost. Always follow the OEM viscosity and API/BIS requirement.
Need an oil built to a specific SAE grade?
From 0W-20 fuel-economy oils to 15W-40 diesel workhorses — we formulate to the exact J300 and API/BIS limits, then prove every number in the lab. The IP is yours.
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