A vehicle's oil reservoir being filled with a funnel. A closer analysis of the numbers on the bottle can tell you more about the oil weight.

Understanding Oil Weight: What Do the Numbers on the Bottle Mean?

Oil weight can be a confusing term. This is partly because “weight” doesn’t actually refer to how a particular lubricant tips the scales. It does, however, have an important bearing on the life of your engine, and the type of oil you choose is intimately linked to manufacturer recommendations, the climate you live in and the type of driving that you do.

Let’s take a closer look at how oil is classified, and how to better understand what the numbers on the bottle actually mean.

Going With the Flow

Oil weight is a term used to describe the viscosity of an oil, which means how well it flows at a specific temperature. The Society of Automotive Engineers (SAE) assigns a viscosity number, or weight, to oil based on its flow at 210 degrees F, which is roughly the standard operating temperature for most motors. The higher the number, the thicker or slower flowing it is, which changes the nature of how it coats internal engine components and protects against heat and friction. This means a 30 weight oil flows more quickly than 50 weight oil, but doesn’t offer quite the same level of protection at higher operating temperatures or in stressful conditions.

What About Winter?

You’ve probably noticed that most bottles of oil actually have two numbers on them, separated by a “w.” These are called multigrade oils, and they are unique in that they have been engineered to offer not just one, but two weights.

Confused? Here’s how it breaks down. The “w” stands for winter and indicates that the lubricant in question has a different viscosity, or different flow characteristics, based on temperature. For example, a 5w30 oil weight rating means that on cold start-up — even at subzero temperatures — the oil flows like a 5 weight oil. However, once warmed up to 210 degrees F, it functions as a 30 weight oil. This is accomplished through the inclusion of unique additives in the mixture.

Oil drainingWhy would you want a thinner oil that acts like a thicker oil once warmed up? If you used a straight oil weight of 30, then the lubricant would be too thick to shoot quickly through the engine when starting on a cold day, which means it would fail to properly protect crucial parts from damage. At the same time, you wouldn’t want to run a straight 5 weight oil either, because it would be too thin to keep a warm engine safe. A multigrade is truly the best of both worlds.

What Oil Should I Use?

You should always use the engine oil weight that is recommended by your vehicle’s manufacturer in the owner’s manual. That being said, most manuals will recommend a range of oils that take into account how harsh your winters might be and whether you’ll be putting extra stress on your engine by towing or hauling a heavy load.

There’s no one-size-fits-all answer to motor oil selection. Just be sure to consult your manual, and choose the oil grade that matches how you drive for the best possible engine protection.

Check out all the chemical products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on engine oil weight, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photo courtesy of Wikimedia Commons.

about author

Benjamin Hunting

Having been bitten by the car bug at a young age, I spent my formative years surrounded by Studebakers at car shows across Quebec and the northeastern United States. Over ten years of racing, restoring, and obsessing over automobiles lead me to balance science writing and automotive journalism full time.  I currently contribute as an editor to several online and print automotive publications, and I also write and consult for the pharmaceutical and medical device industry.

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