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What Is Viscosity?
Viscosity is simply how thick or thin a liquid is. Honey has high viscosity (thick, flows slowly). Water has low viscosity (thin, flows easily). Engine oil viscosity affects how it protects moving engine parts.
Thick oil (high viscosity) creates a strong protective film but flows slowly in cold weather, making cold starts harder. Thin oil (low viscosity) flows easily in cold but may not protect as well at high temperatures.
Understanding SAE Grades
SAE (Society of Automotive Engineers) developed the viscosity grading system used worldwide. Numbers represent viscosity at specific temperatures.
| SAE Grade | What It Means |
|---|---|
| 0W, 5W, 10W, 15W, 20W, 25W | Winter (cold) grades. The “W” stands for Winter. Lower number = thinner at cold temps. 0W flows at -40°C; 5W at -30°C; 15W at -20°C. |
| 20, 30, 40, 50, 60 | Summer (hot) grades. No “W” suffix. Measured at 100°C engine operating temperature. Higher number = thicker at heat. |
Recommended Engine Oils
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Multi-Grade Oils: 5W-30 Explained
Modern cars use multi-grade oils that perform well in both cold and hot conditions. An oil labeled 5W-30 means:
5W: Flows like 5-weight oil when cold. At -30°C (winter morning), it’s thin enough for quick engine start and fast oil circulation.
30: Flows like 30-weight oil at 100°C (normal engine operating temp). Thick enough to protect bearing surfaces and moving parts.
How This Works
The oil contains viscosity index improvers — polymers that expand slightly as the oil warms, thickening it. Cold? The polymers are relaxed, oil is thin. Hot? Polymers expand, oil thickens. One oil works in all conditions.
Common Engine Oil Grades in the UK
| Grade | Typical Use | Cold Performance | Hot Performance |
|---|---|---|---|
| 0W-20 | Modern petrol engines, newer cars | Excellent (-40°C) | Good (20 cSt @ 100°C) |
| 5W-30 | Most common. Petrol and diesel. | Very Good (-30°C) | Good (30 cSt @ 100°C) |
| 10W-40 | Older engines, high-mileage cars | Good (-20°C) | Excellent (40 cSt @ 100°C) |
| 15W-40 | Some diesels, older vehicles | Adequate (-15°C) | Excellent (40 cSt @ 100°C) |
Why Modern Cars Use Thinner Oils
In the 1980s and 1990s, 10W-40 and 15W-40 were standard. Today, 0W-20 and 5W-30 dominate. Why the change?
Fuel Economy
Thinner oil reduces friction between moving parts. Less friction means the engine doesn’t have to work as hard, improving fuel economy by 1-2 mpg.
Cold-Start Protection
Modern engines start in severe cold (-40°C) with 0W oils that old 10W oils couldn’t match. Oil circulates faster to all engine parts before metal-to-metal contact occurs.
Modern Materials
New engines have tighter manufacturing tolerances, better lubricant films, and more efficient bearing surfaces. They don’t need thick oil to survive.
Tighter Emissions
Thinner oil produces lower friction and heat, helping engines meet strict emissions standards.
What the Numbers Really Mean (Technical)
Viscosity is measured in centistokes (cSt) — a unit of kinematic viscosity. Here’s what it means:
5W-30: At -30°C (0°F), the oil must pour like a 5-weight oil. At 100°C (212°F), it must be between 9.3 and 12.5 cSt (that’s 30-weight territory).
Example: A 5W-30 oil at 100°C might be 11.2 cSt. A 10W-40 at 100°C might be 14.5 cSt. The 10W-40 is noticeably thicker at engine temperature.
Synthetic vs Mineral Oil Viscosity
Synthetic oils (fully synthetic or semi-synthetic) maintain viscosity better across temperature extremes than mineral oil. A synthetic 5W-30 outperforms a mineral 5W-30 in extreme cold and at high sustained temperatures.
Cost difference: Synthetic oils cost more upfront but last longer (often 8,000-10,000 miles vs 5,000 for mineral). Over time, the cost per mile is similar or cheaper.
Never Use the Wrong Grade
Your car’s engine is precisely tuned for a specific oil grade. Using the wrong one causes problems:
- Too thick (e.g., 10W-40 when 5W-30 is specified): Cold-start sluggish, fuel economy drops 2-3%, hydraulic systems work harder.
- Too thin (e.g., 0W-16 when 5W-30 is specified): Insufficient protection at high RPM, bearing wear increases, engine life shortened.
Frequently Asked Questions
What does 5W-30 mean?
5W-30 is a multi-grade oil. The 5W refers to cold (winter) viscosity — it flows like 5-weight oil when cold, allowing quick engine starts. The 30 refers to hot viscosity — it flows like 30-weight oil at operating temperature, providing protection for moving parts.
Can I use thicker oil than recommended?
No. Using oil thicker than the manufacturer’s specification increases friction, reduces fuel economy, harms cold-start performance, and may cause insufficient protection at high temperatures. Always use the exact grade specified in your handbook.
Why do modern cars use thinner oils?
Thinner oils (0W-20, 0W-16) reduce engine friction and improve fuel economy. Modern engines have tighter tolerances and better materials that don’t need thick oil protection. Thinner oil also allows faster oil distribution at startup in cold weather.
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How we check fitment for this guide
We cross-reference fitment figures against manufacturer specifications and OE-supplier catalogue data where available, and matched to the standard codes that actually govern fit — battery type/group sizes, wiper lengths and fitting types, bulb fitment codes, tyre size markings, and oil specifications. We work by make, model, year, engine, and build date, because a single model can take different parts across its life. Where the right part depends on your exact vehicle, we tell you to confirm by registration or VIN. More on our method →