What viscosity means
Viscosity is a fluid's resistance to flow. Engine oil must be thin enough to circulate instantly at start-up, yet thick enough to keep a protective film between metal surfaces at full operating temperature. The SAE J300 grading system describes both behaviours in one code.
Reading a multigrade code
In a grade like 5W-30, the number before the W (for 'winter') describes cold-start flow — lower means the oil flows more easily when cold. The number after the dash describes viscosity at operating temperature — higher means a thicker oil film when hot.
- 0W / 5W — very easy cold flow, common in modern fuel-efficient engines.
- 10W / 15W — typical for older engines and hot climates.
- 20 / 30 — lower hot viscosity, often specified for newer engines.
- 40 / 50 — higher hot viscosity, common in older or heavy-duty engines.
Why 'thicker is safer' is a myth
Modern engines are built with tight clearances that assume a specific oil film thickness. Using a much thicker oil than specified can slow lubrication at start-up and increase fuel consumption; much thinner oil can reduce film strength under load. The manufacturer's specified grade is the correct grade.
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