Oil Viscosity Guide
Decode any engine oil grade like 5W-30 — understand the winter and operating viscosity ratings and what they mean for your car.
Oil Viscosity Decoder
Enter grade → full explanation
How It Works
Multi-grade oils have two viscosity ratings: the winter (W) rating measures cold-temperature pumpability; the high-temperature rating measures viscosity at operating temperature. Both are defined by SAE J300.
How to Use This Calculator
- Read the oil grade from your engine oil cap, owner’s manual, or service sticker.
- Type it in the format shown (e.g., 5W-30).
- Click Decode to see what each number means.
- Compare the cold-start temperature to your climate.
- Use the operating viscosity recommendation to choose the right oil for your engine.
Worked Example
Reference Table
| Grade | Cold Limit (W) | Hot Viscosity | Best For |
|---|---|---|---|
| 0W-20 | −35°C | 6.9–9.3 cSt | Modern tight-clearance engines, hybrids |
| 5W-30 | −30°C | 9.3–12.5 cSt | Most petrol engines, standard choice |
| 5W-40 | −30°C | 12.5–16.3 cSt | Turbocharged petrol, European diesels |
| 10W-40 | −25°C | 12.5–16.3 cSt | Older engines, hot climates |
| 15W-40 | −20°C | 12.5–16.3 cSt | Diesel trucks, older engines |
| 0W-40 | −35°C | 12.5–16.3 cSt | High-performance Euro engines (AMG, BMW M) |
Frequently Asked Questions
Should I use thicker oil in a high-mileage engine?
Some mechanics recommend stepping up one viscosity grade (e.g., 5W-30 to 5W-40) in engines over 150 000 km with worn bearings and seals. However, always check the manufacturer’s specification first — many modern engines are not tolerant of higher-viscosity oils.
What does ‘cSt’ mean and why does it matter?
cSt (centistokes) measures kinematic viscosity — how easily the oil flows. At 100°C, higher cSt means the oil is thicker under operating conditions, providing a better film on worn surfaces but requiring slightly more energy to pump.
Can I mix different viscosity grades of the same brand?
Mixing grades of the same oil type (e.g., 5W-30 and 5W-40 of the same brand) is acceptable for a top-up in an emergency. The resulting viscosity will be between the two grades. It is not recommended for regular use.
Why do electric vehicles not need engine oil?
EVs have electric motors with no internal combustion, so no piston-cylinder lubrication is needed. EVs do use other lubricants: gear oil in the reduction gearbox and thermal management fluid in the battery cooling system.
