What Engine Oil Grades Actually Tell You
Walk into any auto parts shop in Delhi and you’ll see bottles labelled 20W40, 10W30, 15W40, 5W30 and more. For most vehicle owners these numbers look like a code — and picking the wrong one can quietly shorten your engine’s life. This guide has engine oil grades explained in plain language, so you know exactly what those figures mean before you buy.
An engine oil “grade” is really a viscosity rating set by the Society of Automotive Engineers (SAE). Viscosity is simply how thick or thin the oil is, and therefore how easily it flows and how well it clings to metal parts to form a protective film. Get it right and the oil pumps quickly at start-up and stays strong at operating temperature. Get it wrong and you risk poor lubrication, higher fuel consumption and faster wear.
Breaking Down the Numbers: 20W40, 10W30 and 15W40
Almost every modern engine oil is a multigrade oil, which is why you see two numbers separated by a “W”. Here is how to read them:
The first number and the “W”: The W stands for “Winter”, not weight. The number before it — the 10 in 10W30, or the 15 in 15W40 — describes how the oil behaves when it is cold. A lower number means the oil stays thinner in the cold and flows faster the moment you crank the engine, which matters most during a cold start when most engine wear happens.
The second number: The figure after the W — the 40 in 20W40, or the 30 in 10W30 — describes the oil’s thickness at full operating temperature, around 100°C. A higher second number means a thicker film at heat, which protects better under heavy load and high temperatures but can slightly reduce fuel efficiency.
So a 10W30 is thinner both when cold and hot than a 20W40. In practical terms, 10W30 flows more freely and suits engines built to tighter tolerances, while 20W40 offers a heavier protective film that many older Indian petrol engines and hard-working vehicles prefer. A 15W40 sits in between and is the workhorse grade for most diesel cars, trucks and tractors. If you want a full side-by-side, our guide on 20W40 vs 10W30 for Delhi traffic breaks the choice down further.
Why the Right Grade Matters in Indian Conditions
India is not a single climate, and that changes what “correct” looks like. Summer temperatures in Delhi routinely cross 45°C, while northern winters and hill stations can drop close to freezing. Stop-and-go city traffic keeps engines hot and idling for long spells, and dust is everywhere. All of this stresses the oil film.
In consistently hot conditions and heavy traffic, an oil with adequate high-temperature viscosity — the second number — helps maintain protection when the engine is baking. In colder regions, a lower cold-rating first number ensures the oil reaches critical parts quickly on a freezing morning. Choosing a grade tuned to real Indian conditions, rather than simply copying a number from an unrelated market, is one of the easiest ways to extend engine life.
How to Choose the Correct Engine Oil Grade
The single most important step is to check your owner’s manual. Vehicle manufacturers specify a recommended SAE grade based on the engine’s design tolerances, and staying within that recommendation protects both your engine and, in many cases, your warranty.
Beyond the manual, keep these points in mind:
- Match the grade to the engine, not the price tag. A thicker oil is not automatically “better” — using 20W40 where 10W30 is specified can increase drag and fuel use.
- Consider the vehicle’s age and mileage. Higher-mileage engines with more internal clearance sometimes benefit from a slightly heavier grade to maintain oil pressure.
- Look past the grade to the specification. The API service rating (such as SN or CI-4) and any OEM approval matter just as much as viscosity. Our post on API ratings explained covers this in detail.
- Buy from a manufacturer you can verify. Grade markings only help if the oil in the bottle genuinely meets them.
The Bottom Line
Once you know that the first number is about cold flow, the “W” means winter, and the second number is about hot-temperature protection, engine oil grades stop being a mystery. 10W30, 15W40 and 20W40 are not interchangeable — each is engineered for a particular balance of flow and film strength, and the right one for you is the grade your engine was designed around and that suits your driving conditions.
Petroblast Lubricants manufactures engine oils, gear oils, greases and DEF across the grades Indian vehicles actually run on, right here in Mundka, New Delhi. Browse our full range on the Products page, or get in touch if you’d like help matching a grade to your fleet or private-label requirement.