How Oil Grades Work
Engine oil grades use the SAE (Society of Automotive Engineers) system. The format is XW-Y where:
- X = cold viscosity (W stands for Winter). Lower number = better flow in cold weather.
- Y = hot viscosity at 100°C. Higher number = thicker oil film at operating temperature.
For example, 5W-30 flows well at -35°C (the 5W part) and provides moderate protection at 100°C (the 30 part).
Common Grades in the UK
| Grade | Cold Flow | Hot Protection | Typical Use |
|---|---|---|---|
| 0W-20 | Excellent (-40°C) | Light | Modern fuel-efficient engines (Toyota, Honda, VW) |
| 0W-30 | Excellent (-40°C) | Medium | Premium European cars (BMW, Mercedes, Volvo) |
| 5W-30 | Very good (-35°C) | Medium | Most common UK grade — works for most cars |
| 5W-40 | Very good (-35°C) | Higher | Performance/older European engines |
| 10W-40 | Good (-25°C) | Higher | Older engines, semi-synthetic option |
Which Grade Do I Need?
Always use the grade specified in your vehicle handbook. Modern engines are designed around specific oil viscosities — using a different grade can:
- Void your manufacturer warranty
- Increase fuel consumption
- Cause engine wear or damage
- Clog your DPF (diesel vehicles)
Frequently Asked Questions
Can I use 5W-30 instead of 0W-20?
Only if your manufacturer allows it. Many modern engines (Toyota, Honda, VW) specifically require 0W-20 for fuel economy and emissions. Using 5W-30 will make the engine work harder and may trigger warning lights.
Is thicker oil better for older engines?
Not necessarily. Stick to the manufacturer’s recommendation. If your engine burns oil, consult a mechanic rather than switching to a thicker grade.
Does the brand of oil matter?
As long as the oil meets the correct grade AND manufacturer specification (e.g., VW 508 00, BMW LL-04), any reputable brand is fine.
