Engine Oil Grades

Engine oil grades, indicated by codes like 5W-30 or 10W-40, define viscosity and performance in different temperatures. The first number (e.g., 5W) shows cold-weather flow (lower = better in winter), while the second (e.g., 30) indicates high-temperature thickness. For most modern cars, 5W-30 is common, but always check your handbook.

1. Cold climates: A 0W or 5W grade ensures easier cold starts.
2. High-mileage engines: Thicker oils like 10W-40 reduce wear in older engines.
3. Performance cars: Synthetic 5W-40 or 10W-60 may be specified for high-stress engines.

Using the wrong grade risks poor lubrication or reduced efficiency. For example, a Ford Focus typically uses 5W-20, while a BMW M3 might require 10W-60. Synthetic oils (e.g., Shell Helix) offer better longevity and protection.

From a mechanic’s view: Oil grades matter because they affect engine wear. Thinner oils (e.g., 0W-20) flow faster at startup, protecting turbochargers in cars like the Nissan Leaf. Thicker grades (15W-50) suit older engines with wider tolerances. Always match the manufacturer’s recommendation—using 10W-40 in a Tesla Model 3 could void the warranty.

Vehicle engine oil grades are classified based on two key indicators: SAE and API. The code following SAE denotes the oil's viscosity rating, while the code after API indicates the oil's quality grade.

On engine oil packaging, you'll commonly see both SAE (the abbreviation for Society of Automotive Engineers) and API (the abbreviation for American Petroleum Institute).

Classification by SAE viscosity grades

Engine oils are divided by SAE viscosity grades into 6 winter grades, 5 summer grades, and 16 all-season grades.

1. The 6 winter grades are 0W, 5W, 10W, 15W, 20W, and 25W. The 'W' stands for Winter, and a lower preceding number indicates better fluidity at lower temperatures.
2. The 5 summer grades are 20, 30, 40, 50, and 60. Higher numbers indicate greater viscosity and better high-temperature protection for warmer climates.
3. The 16 all-season grades are: 5W20, 5W30, 5W40, 5W50, 10W20, 10W30, 10W40, 10W50, 15W30, 15W40, 15W50, 20W20, 20W30, 20W40, and 20W50.

Classification by API quality grades

The API quality system divides oils into S-series and C-series. The S-series denotes petrol engine oils, while C-series denotes diesel engine oils.

When marked with both S and C, the oil is suitable for both petrol and diesel engines. Each series uses letters (A, B, C, D, E, F, etc.) for grading - later letters indicate higher quality grades with better performance.

Current petrol engine oil grades are SA, SB, SC, SD, SE, SF, SG, SH, SJ, SL, SM, SN, and SP. Each successive letter indicates improved performance with more protective additives for engines - later letters denote higher quality grades.

Diesel engine oil grades include CA, CB, CC, CD, CD-II, CE, CF-II, CF-4, CG-4, CH-4, and CI-4.