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Engine Oil Viscosity Guide
Decode engine oil grades such as 5W-30: what each number means, how climate affects the choice, and why the manufacturer's spec matters most.
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Engine Oil Viscosity Guide
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Grade explained
5W-30: the 5W is its cold-start behaviour (W for winter), the 30 its viscosity at 100 °C. Against 10W-40 it flows better on a cold morning and is thinner when hot.
How the Engine Oil Viscosity Guide works
- Select the grade printed on the bottle or in your handbook.
- Read what each half of the grade describes — they measure two different temperatures, not a range.
- Check the manufacturer's specification alongside viscosity; the approval code matters as much as the numbers.
The method
A multigrade oil is rated twice: once for low-temperature pumpability and once for viscosity at operating temperature.
5W-30 = winter rating 5, high-temperature viscosity grade 30 at 100 °C
A lower W number flows more readily at start-up, when most engine wear occurs. The second number is not a range with the first — they are separate measurements.
FAQ
Can I use a thicker oil in an old engine?
It is a common habit and occasionally justified, but modern engines with variable valve timing and tight oil galleries depend on the specified grade. Deviating can disable systems that rely on oil pressure and flow.
What do the approval codes mean?
Codes such as API SP or an OEM approval describe the additive package and performance testing, not just viscosity. An oil of the right grade but the wrong specification can still be unsuitable.
Does synthetic matter more than grade?
They are different questions. Synthetic bases hold viscosity better across temperature and last longer, but the grade and the manufacturer's approval remain the requirement.
How we compare
| Feature | Online Tool Store | Pen and paper | A classroom platform |
|---|---|---|---|
| Explains both numbers | ✓ | Partially | Yes |
| Covers approval codes | ✓ | ✗ | ✓ |
| Manufacturer-neutral | ✓ | ✓ | ✗ |
| Free to read | ✓ | ✓ | ✓ |
Engine Oil Viscosity Guide explains the grade, then points at the manufacturer's approval code — which is the part that decides whether an oil is actually suitable.