Oil Viscosity: What It Means and How It Affects Your Engine
When you see numbers like 5W-30 or 10W-40 on a bottle of oil viscosity, a measure of how thick or thin engine oil flows at different temperatures. Also known as oil grade, it tells you how well the oil protects your engine in cold starts and under heavy heat. This isn’t just marketing jargon—it’s the difference between your engine lasting 200,000 miles or failing at 80,000.
Think of oil viscosity like honey. Cold honey is thick and slow to pour—that’s what happens when oil is too thick in winter. Too thin, and it won’t stick to engine parts when it’s hot. The first number (like 5W) tells you how it flows when cold. The second number (like 30) shows how it holds up when the engine hits 100°C. If you use the wrong viscosity, your engine doesn’t get the right protection. Too thin, and metal parts grind together. Too thick, and the engine works harder, burning more fuel and overheating faster.
Motor oil, the liquid that lubricates, cools, and cleans your engine’s moving parts, changes over time. It breaks down, picks up dirt, and loses its ability to maintain proper viscosity. That’s why regular oil changes aren’t optional. Skipping them lets sludge build up, which clogs oil passages and starves critical parts like the camshaft and pistons. Your car’s manual doesn’t just suggest a viscosity—it demands it. Using 10W-40 when it calls for 5W-30 might seem harmless, but in the UK’s wet, stop-start traffic, that mismatch can cause premature wear.
Even small changes in oil thickness, how easily oil moves through engine channels under pressure affect performance. A thicker oil might seem like better protection, but modern engines are designed with tight tolerances. They need oil that flows quickly on startup to prevent dry friction. That’s why synthetic oils have become standard—they hold their viscosity better under extreme heat and cold than conventional oils. If your car’s been tuned, modified, or driven hard, you might need a different viscosity than stock. But never guess. Check your manual, or ask a mechanic who knows your exact model.
And don’t confuse oil viscosity with oil quality. A high-viscosity oil isn’t automatically better. A cheap 10W-40 can break down faster than a premium 5W-30. What matters is matching the right viscosity to your engine’s design, your climate, and your driving habits. If you live in London and do short trips, your oil never fully warms up. That means water builds up, and viscosity control becomes even more critical. If you drive on A-roads or motorways regularly, heat builds up—and your oil needs to resist thinning out.
Below, you’ll find real-world guides on what happens when you ignore oil viscosity, how to spot oil-related engine problems, and why topping up isn’t the same as changing. Whether you’re maintaining a daily driver or hunting for parts at auction, understanding this one number can save you thousands.
