With all of the options available when it is time to purchase motor oil, how do you decide what to buy? Should you go for a particular brand? Or a certain type? Is conventional oil just as good as synthetic? Is one grade of oil better than another? And what does “grade” even mean? Let’s take a look at some commonly asked questions about motor oil and see if the answers can clear up some confusion.
When it comes to motor oil, what is a “grade”?
To start with, we should clear up some confusion over motor oil (also called “engine oil”) terminology. Motor oil grades are a scale developed by the Society of Automotive Engineers (SAE) to describe the viscosity of oil. Viscosity is the ability of a liquid to resist flow – how thick it is. Oil grades relate to an oil product’s performance at operating temperature (about 212 degrees Fahrenheit in your engine). You might be familiar with terms such as “30-weight” or “10W-30” oil. Those numbers refer to the grade. The higher the number, the thicker the oil. The lower the number, the thinner. Thinner oils flow faster than thicker oils, and engineers design engines to work with a certain grade of oil.
For all practical purposes, the terms “grade” and “viscosity” refer to the same thing with motor oil. So does the term “weight”. Each of these words are used interchangeably when people talk about motor oil. So, SAE30 oil is also 30-weight, and both describe the oil’s viscosity or thickness.
What is the difference between “straight-weight” and “multi-weight” (multi-viscosity) oil?
Many years ago, straight-weight motor oils were common. Straight-weight oils are lubricants that have a set viscosity, a single grade. SAE50 is higher viscosity (thicker) than SAE40, and so on. Straight-weight oils come in a wide range of viscosities.
But your car operates (ostensibly) in a wide range of temperatures. When your engine is running at operating temperature, it does not matter too much if it is summer or winter. The engine is still warm and so is the oil. But if your car has been sitting all night, the difference in temperature can be very different from one season to another.
Why is this important? Because oil gets thinner as it heats up. Or you could say it gets thicker when it cools down. Say your engine needs 30-weight oil when it is running at operating temperature. Well, 30-weight oil will work perfectly well when your engine is warmed up. But when you go to start it on a cold winter morning, that 30-weight oil is considerably thicker and has a hard time moving when you turn the key.
Enter “multi-weight” oil. These products are one grade or viscosity (or weight) when cold and another when warm. You have seen 10W-30, 5W-20, and the like. The first number refers to the grade when cold and the second when warm. Actually, the oil itself is the lower grade and contains additives that make it resist thinning out as it gets hot, thereby making it work like a higher-viscosity oil. So, 5W-20 is a 5-weight oil that acts like 20-weight when warm.
What does the “W” stand for in a multi-weight oil?
So the numbers represent the oil’s grades when cold and when warm. But what about the letter “W”? Simple. It stands for “Winter”.
When might it make sense to use straight weight oil (ie. SAE30) rather than multi-weight oil (5W-30)? Straight-weight oil has one potential advantage over its multi-weight cousins. Because it does not contain viscosity improvers (the additives that allow a low-viscosity oil to perform as a higher-viscosity oil), straight-weight oil has better shear protection. Shear is the breakdown of viscosity during operation, especially when an engine is running at high RPMs. Some muscle cars with older engines might benefit from straight-weight oil unless they are driven in the cold, in which case the benefit of a multi-weight oil outweighs the improved shear resistance. Most engine wear occurs not when the engine is hot, but on startup when the engine is cool. If you drive a classic, you might consider straight-weight.
That said, straight-weight oil is never recommended for an engine that calls for multi-weight oil.
Is thicker oil better for an engine?
There are some circumstances where drivers have used thicker oil in an engine. If, for instance, clearances between engine components have increased, gotten sloppy, a thicker oil can help to fill the void. Within reason, thicker oil maintains a better lubricant film between moving parts. Some have even used thicker oil in a leaky engine to prevent oil from seeping out.
But really, thicker oil is not good for your engine. Not when “thicker” means higher viscosity than the manufacturer recommends. Your engine was built to specific tolerances – spaces between the moving parts. So the recommendation for a specific grade of oil is deliberate. The oil needs to be able to coat those surfaces, but it also needs to be able to flow into all of the tight spaces in a modern engine that has been engineered to be smaller and lighter weight, and with tighter tolerances. The best grade of oil for your engine is the grade recommended by the manufacturer.
Is it okay to switch grades of motor oil, say, from 5W-20 to 10W-30?
With that in mind, some manufacturers suggest only one grade of motor oil for use in a particular engine, while others set a range of grades based on the climate in which the vehicle is driven. If that is not the case with your car, stick with the manufacturer’s recommendation.
How about 5W-30 instead of 5W-20?
Again, although both of these examples are 5-weight oil when cold, they have different properties when warm. Using oil that is thicker than recommended may lead to a decrease in fuel economy, a higher load on your engine, and even a shorter life for your engine. Conversely, using thinner, lighter-weight oil than recommended can cause excessive wear and shorter life. Follow the manufacturer’s instructions.
Columbia Auto Care & Car Wash | Author: Mike Ales | Copyright November 2019
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