What your engine oil's SAE grade means, and which to buy in India

What the two numbers on the bottle mean

Every bottle of engine oil carries a code like 5W-30 or 15W-40. This is not a brand's invention. It comes from SAE International's standard J300, Engine Oil Viscosity Classification, now in its May 2024 revision, J300_202405, published by the society's Fuels and Lubricants technical committee.

The first number, followed by W, describes how the oil behaves cold. SAE J300 sets a maximum cold cranking simulator viscosity for each W grade at a fixed test temperature. A 0W oil must stay thin enough at minus 35 degrees Celsius, a 5W at minus 30, a 10W at minus 25, a 15W at minus 20, a 20W at minus 15 and a 25W at minus 10. The lower that temperature, the easier the oil flows to the bearings on a cold start.

The second number, measured without a W, is the oil's kinematic viscosity at 100 degrees Celsius, the temperature a running engine reaches. The standard's table gives each grade as a band from a stated minimum up to, but not including, a stated maximum. SAE 20 runs from 6.9 to just under 9.3 centistokes, SAE 30 from 9.3 to just under 12.5, SAE 40 from 12.5 to just under 16.3 and SAE 50 from 16.3 to just under 21.9. A thinner number here means less drag and better fuel economy. A thicker number means a stronger oil film when the engine is working hard.

Why SAE rewrote the standard in 2024

The 2024 revision added something the old tables lacked, a minimum limit for cold viscosity, not just a maximum. Until then, an oil near a grade boundary could sometimes be sold under more than one SAE number, because only the upper limit was policed. The new table closes that gap.

The committee also leaned harder on a second measurement called high temperature, high shear viscosity, or HTHS, taken at 150 degrees Celsius under load. This matters most for the newest ultra thin grades, SAE 8, 12 and 16, where the kinematic viscosity bands sit close to the familiar SAE 20. HTHS stops a thin oil from being sold as a slightly thicker one just because it scrapes past the 100 degree test. These very thin grades exist mainly for low friction hybrid and turbo petrol engines sold in Japan, Europe and North America, chasing fuel economy limits, and have not reached most Indian dealerships yet.

What it means for buying oil in India

For an Indian buyer, the cold start number rarely matters on its own terms. Even a Shimla or Srinagar winter morning seldom drops to the minus 20 or minus 30 degrees these grades are tested at. Carmakers still specify 0W or 5W oils for Indian cars because the same global engine calibration is tuned around that grade for fuel economy and emission certification, not because Delhi needs it to start.

The real trade-off plays out at the other end. A thinner oil such as 0W-20 gives a small mileage gain and less load on the oil pump, but needs a genuine full synthetic base and a healthy HTHS number to hold its film through Bengaluru's stop and go traffic or a long Mumbai summer in the high thirties. A thicker oil such as 15W-40 or 20W-50, still common in older petrol and diesel cars and most two wheelers in India, trades a bit of fuel economy for a wider margin of protection in heat and gives more leeway if service intervals slip.

The grade printed in the owner's manual already accounts for this balance for that specific engine. Switching to a thicker oil than specified, a habit some mechanics still recommend before a long drive, does not add safety margin on a modern engine tuned for a thinner one. It mostly adds drag. The safer move is to match the manual's SAE number and buy oil that actually meets it, since a grade is only as good as the base oil and additive package behind the two numbers on the label.

Sources

• SAE J300_202405, Engine Oil Viscosity Classification
• SAE J300 engine oil viscosity, 2024 update explained
• SAE J300 viscosity grade table