Oil Grades- What do they mean?

Posted on: July 20th, 2012 by sara


We’ve all seen or heard of 5w-30, 10w-40 and SAE 40 before, but for many of us we have no clue what those Viscosity Grades mean -or even that they are called viscosity grades. Let me act as your decoder ring. In this blog I will give you a little background on grades. I’ll explain how they come up with those wacky numbers and explain the basic differences between the grades.

 Before I go into all the great benefits of different oils for your vehicle, let me first explain what motor oil does for your engine.

Motor oil lubricates engine components and cuts down on friction. Friction causes increased heat (which increases wear) and the engine parts to slow down. Motor oil creates a small barrier or film between the parts in order for them to slide past one another. The slippery portion of parts increases efficiency, power and performance. Another important thing to note is that a properly lubricated oil uses less fuel than one that isn’t properly lubricated.

One more piece of the puzzle to consider: when the engine isn’t running, oil drips off the engine. That’s why the majority of engine wear comes from start up. Less lubricant on the engine = more friction and More friction = more wear.

With those factors in mind, let’s talk about engine oil differences.

The main difference between basic oils is their viscosity grades. Don’t let the word overwhelm you, it is really quite simple. Viscosity is a measurement of how quickly the oil flows through your engine. Since thicker or higher viscosity oils take longer to flow, it makes sense that an oil with a higher viscosity grade would run more slowly through the engine components. This means that SAE 30 moves faster than SAE 60, for example.

Oils with a higher viscosity provide more lubrication and a greater level of protection to your engine. The downside is that these oils also take longer to initially cover the engine components after vehicle start up. And remember; start up will cause greater wear on your engine than the general running of the engine.

Oil with a lower viscosity provides a quicker flow of oil to cover the engine components, thus minimizing the amount of dry running. However, lower viscosity oils provide less lubrication during normal running of the engine.

So what are you to do- protect your engine at start up or protect your engine while it is running? Luckily, it doesn’t have to be either or. Some oils react differently in different temperatures. These oils have two numbers in its viscosity grade. You are probably familiar with 5W-30 or 10W-40, they are examples of these types of oils. 

Let’s look a little more closely at 5W-30. Anytime you see a number with a W,  it represents how the oil will react in colder temperatures (when the engine is just turning over versus when it has been running for a while). As you can see, it has a very low W viscosity grade. That allows a quick flow of oil when the vehicle is first started. This coats the engine components quickly so there is less dry rubbing. Then as the temperature of the oil increases it becomes thicker and more lubricative.

So there you go! The mystery of oil viscosity is now solved!