What Do the Numbers Actually Mean?
Engine oil viscosity grades follow the SAE (Society of Automotive Engineers) standard. Most modern engine oils are multi-grade, meaning they behave differently at cold temperatures versus operating temperatures.
Take 5W-30 as an example. The "5W" is the winter rating — it tells you how the oil flows when cold. The lower this number, the better the oil flows at low temperatures and the faster it reaches critical engine parts on a cold start. The "30" is the viscosity at operating temperature (100°C) — higher numbers mean thicker oil at heat.
Key insight: Most engine wear happens in the first 10–30 seconds after startup, before the oil fully circulates. Cold-start flow matters enormously — which is why the "W" rating is critical for engines in cold climates or that sit idle overnight.
How SAE J300 Defines the W and Hot Grades
The grade on the pack is set by laboratory limits in the SAE J300 standard, and the two halves of a multigrade are tested differently. The W number is earned at sub-zero temperatures: the cold-cranking simulator (CCS) checks the starter can turn the engine over, and the mini-rotary viscometer (MRV) checks the pump can still draw oil to the bearings.
| W grade | Cranking limit (CCS) | Pumping limit (MRV) |
|---|---|---|
| 0W | 6,200 mPa·s max at −35 °C | 60,000 mPa·s max at −40 °C |
| 5W | 6,600 mPa·s max at −30 °C | 60,000 mPa·s max at −35 °C |
| 10W | 7,000 mPa·s max at −25 °C | 60,000 mPa·s max at −30 °C |
| 15W | 7,000 mPa·s max at −20 °C | 60,000 mPa·s max at −25 °C |
| 20W | 9,500 mPa·s max at −15 °C | 60,000 mPa·s max at −20 °C |
The number after the dash is a band of kinematic viscosity at 100 °C, plus a minimum HTHS (high-temperature high-shear) viscosity at 150 °C, close to what a loaded crankshaft bearing actually sees.
| Hot grade | Kinematic viscosity at 100 °C | Minimum HTHS at 150 °C |
|---|---|---|
| SAE 20 | 6.9 to under 9.3 cSt | 2.6 mPa·s |
| SAE 30 | 9.3 to under 12.5 cSt | 2.9 mPa·s |
| SAE 40 | 12.5 to under 16.3 cSt | 3.5 mPa·s for 0W-40, 5W-40 and 10W-40; 3.7 mPa·s for 15W-40, 20W-40 and straight 40 |
| SAE 50 | 16.3 to under 21.9 cSt | 3.7 mPa·s |
Indian mornings rarely come near those test temperatures, so across most of the country the W number governs how quickly oil reaches the top end on a cool start, not whether the engine cranks; it matters most in the hills and north Indian winters. Note too that 15W-40 and 20W-40 sit in the same 100 °C band; the difference between them shows up mainly below full operating temperature, where the 20W-40 is the thicker oil. Our 15W-40 vs 20W-40 guide covers that choice in detail.
The Common Grades and When to Use Each
| Grade | Best For | Climate |
|---|---|---|
| 0W-30 | Modern petrol engines, fuel-efficient vehicles, hybrids | Cold climates, all-season |
| 5W-30 | Most passenger cars, SUVs, light commercial vehicles | Temperate to warm climates |
| 10W-30 | Older petrol engines, warmer operating environments | Warm to hot climates |
| 15W-40 | Diesel trucks, commercial fleets, tractors, older engines | Hot climates, heavy load |
| 20W-40 | High-mileage engines, older vehicles, very hot climates | Tropical / extreme heat |
| 4T | 4-stroke motorcycle and 2-wheeler engines | All climates |
0W-30 — Modern Engines, Cold Starts
The thinnest cold-weather grade in common use. Designed for modern, high-precision engines with tighter tolerances. Provides excellent fuel economy and near-instant lubrication on cold starts. Not recommended for older engines with larger clearances — it may not maintain sufficient film strength under load.
5W-30 — The All-Rounder
The most widely specified grade in passenger car OEM recommendations. Balances cold-flow performance with adequate high-temperature viscosity. If your vehicle manual specifies 5W-30 and you're operating in India's climate, this is typically the right choice for cars under normal conditions.
15W-40 — The Fleet and Commercial Standard
The workhorse of the commercial transport and agriculture sector. Thicker at operating temperature, making it suitable for high-load diesel engines, turbocharged trucks, and farm equipment running long hours in hot conditions. Haruto NexaGuard 15W-40 is offered for exactly this duty: diesel fleets, trucks and tractors.
20W-40 — High Mileage and Tropical Use
Suited to engines with worn seals or larger clearances — the higher viscosity compensates for internal wear. Also useful in extreme ambient temperatures above 45°C where thinner oils may thin out under sustained load.
What Indian Operating Conditions Do to Engine Oil
Indian duty cycles push oil harder than the global average a manual is written for, and each one moves viscosity in service:
- Heat and load together. A loaded truck on a ghat in May or a tractor on a rotavator at midday runs the sump far hotter than highway cruising. Hotter oil is thinner, so the hot number and HTHS matter more than the W number.
- Long idling. Lightly loaded gensets and trucks queued at toll plazas run cool, letting fuel and soot reach the sump. Fuel thins the oil, soot thickens it; either way it drifts out of grade.
- Stop-start city running. Short trips never get the oil fully hot, so water and unburnt fuel do not boil off. A milky film under the filler cap in the monsoon is the warning.
- Dust. Dust that slips past a damaged air filter or loose hose on quarry, site and village-road work turns any grade into a grinding paste and thickens it.
- Overloading. Payload above the rating raises bearing loads and oil temperature, thinning the film the manual's grade was chosen to provide.
Plan for severe service. None of this is a reason to go thicker than the manual. If your vehicles idle, run overloaded, work in dust or do short trips, use the manual's severe-duty drain interval and check the oil more often.
Signs the Viscosity Is Wrong for the Engine
| Too thin for the duty (or diluted by fuel) | Too thick for the engine (or loaded with soot) |
|---|---|
| Low oil pressure reading or warning light at hot idle | Slow, laboured cranking on cool mornings |
| Rising oil consumption between drains, blue smoke on the overrun | Pressure stays high long after start-up; lifter or tappet noise just after starting |
| Louder valve-train or bearing noise once fully hot | Higher fuel consumption with no other explanation |
| Oil on the dipstick looks watery or smells of fuel | Oil turns black and sticky well before the drain is due |
One symptom alone proves little; a worn pump or faulty pressure sender gives similar readings. If the signs persist after a fresh fill of the correct grade, the engine needs mechanical diagnosis, not a different oil. Used-oil analysis confirms in-service viscosity, fuel dilution and soot.
Three Questions to Choose the Right Grade
1. What does your OEM manual specify? Always start here. Vehicle manufacturers test their engines with specific grades and publish the correct specification. Deviating from this — especially going thicker — can reduce fuel efficiency and increase engine stress in modern engines.
2. What is your operating climate? In most parts of India where ambient temperatures range from 15°C to 45°C, 5W-30 or 15W-40 covers most applications. For fleets operating in hill stations or colder regions, the lower "W" rating becomes more important.
3. What is the age and condition of your engine? Older engines with higher mileage often benefit from a slightly higher viscosity grade — the additional thickness compensates for internal wear and reduces oil consumption.
Common mistake: Many fleet operators default to 20W-50 thinking "thicker is safer." In modern engines, this is often counterproductive — the oil is too thick to flow quickly at startup, and the added drag reduces fuel efficiency without protecting the engine better.
Viscosity, Fuel Economy and Wear: The Real Trade-off
Thinner oil takes less energy to shear, so it lowers friction and helps fuel economy; that is why carmakers now specify 0W and 5W grades. The gain is real but modest, visible across a fleet's annual fuel bill rather than one tank, and only in engines designed for it.
The cost is film thickness. A heavy diesel's bearings and rings are designed around a minimum HTHS, so a thinner grade trades a small fuel saving for wear. Thin oil also slips past rings and valve-stem seals, and lighter base oils evaporate faster when hot (Noack volatility on a data sheet), so consumption climbs. Run the thinnest grade your engine maker approves for your conditions, and no thinner.
When the Manual Allows More Than One Grade
Many manuals print a temperature chart rather than a single grade, with several grades each shown against an ambient range. Read it in this order:
- Find the coldest morning the vehicle will start in and choose a W number that covers it.
- Find the hottest working day and check the hot number covers that too.
- Check the performance category printed beside the chart, such as an API or ACEA level or the maker's own approval. Grade and category must both be met.
- For a mixed fleet, choose the grade approved for the most engines, so the store holds fewer oils and top-up mix-ups drop.
A grade that is not on the chart is not an approved alternative, however close it looks.
Topping Up and Mixing Grades
Engine oils of the same type are generally compatible, so mixing grades will not cause sludge. The result has an in-between viscosity and the performance of the weaker oil. Practical rules:
- Top up with the grade and type already in the engine. Keep a 1 L or 5 L pack of it in the vehicle or at the depot.
- If you are low on the road and only another grade is available, adding it is far better than running below the minimum mark. Change to the correct grade at the next opportunity.
- Mineral and synthetic oils of the same grade mix safely too; see mineral vs synthetic engine oil for when the upgrade is worth it.
- If you are adding more than the manual allows between drains, the engine is burning or leaking oil; a thicker grade only hides it.
The Role of Additive Quality
Viscosity grade is only half the equation. A 5W-30 from a premium formulation and a 5W-30 from a budget brand carry the same grade label — but the additive packages differ significantly. Anti-wear agents, detergents, dispersants, and oxidation inhibitors determine how long the oil maintains its performance between drain intervals and how well it protects under sustained load.
The Haruto NexaGuard Engine Oil Range uses a fully formulated additive system that ensures strong film strength, thermal stability, and cleanliness across all available viscosity grades.
How to Read the Pack Label and Data Sheet
The front of the pack gives the SAE grade and usually the performance category, such as API CI-4 or CK-4 for diesel engines or JASO MA for wet-clutch motorcycles. The product data sheet (PDS) gives the numbers behind the label. These are the lines worth checking:
| Data sheet line | What it tells you | What to check |
|---|---|---|
| Kinematic viscosity at 100 °C (KV100) | Thickness at operating temperature | It falls inside the J300 band for the hot grade: 12.5 to under 16.3 cSt for any 40 grade |
| Kinematic viscosity at 40 °C (KV40) | Thickness during warm-up | Compare products of the same grade; a higher KV40 means a thicker oil while warming up |
| Viscosity index (VI) | How little viscosity changes with temperature | Higher means steadier behaviour from cold start to hot running |
| HTHS viscosity at 150 °C | Film strength in loaded bearings | At or above the J300 minimum and any figure the engine maker sets |
| Pour point | Lowest temperature at which the oil still flows | Matters in cold regions; far below any temperature on the Indian plains |
Ask for the PDS before buying in drums, and avoid a supplier whose KV100 falls outside the J300 band for the grade on the label. For how the diesel categories differ, see API CI-4 vs CH-4 vs CK-4.
A Quick Decision Checklist
| Your situation | What to do |
|---|---|
| The manual names one grade and category | Use exactly that, and stop here |
| The manual gives a temperature chart | Pick the listed grade that covers your coldest start and hottest working day |
| Modern petrol car specifying 0W or 5W | Stay on the thin grade; going thicker costs fuel and some engines calibrate valve timing around it |
| Diesel truck, bus, tractor or genset | Typically 15W-40 or 20W-40 at the diesel category the maker lists |
| Wet-clutch motorcycle | A 4T oil meeting JASO MA |
| Severe duty: dust, idling, overloading, short trips | Same grade, shorter drain interval |
| High-mileage engine using oil | Check for wear and leaks first; go one step thicker only within the manual's chart |
Haruto NexaGuard engine oils come in 0W-30, 5W-30, 15W-40, 20W-40 and 4T grades, in packs from 500 ml to 210 L. See the engine oil range and request the data sheet for your grade.