Does Costco's Kirkland Signature Motor Oil Pass The Same Tests As Other Brands?

Costco can do a pretty good job helping drivers save money, whether the company is offering a good deal on tires, or discounts on brand-new Corvettes, or a low-cost motor oil under its own house label, Kirkland Signature. Yes, that's the same brand slapped on Costco krill oil, olive oil, and vegetable oil. And Costco underwear, protein bars, snack nuts, baby wipes, and more. Much more.

But that doesn't mean there's anything wrong with any of those products. Just ask the millions of people who devour the Kirkland Signature rotisserie chickens each year.

So it should come as no surprise that Kirkland Signature motor oil can pass the same tests as its brand-name competition. The same way Costco's cooked chicken has to meet certain guidelines to keep people healthy, Costco's house-brand motor oil — like everybody else's — is held to strict standards to help maintain your car's health.

Multiple agencies set standards for motor oil, including the Society of Automotive Engineers (SAE) and the Petroleum Quality Institute of America (PQIA). The former, for example, is behind the common ratings for oil, like "SAE 5W 30," where the "5" represents how thick the oil is during a cold start and the "30" is how thick it is at regular operating temperatures. (The "W" means it's suitable for winter driving.)

The PQIA is a good source for finding more details with its easy online lookup tool. Using that to compare Kirkland Signature with, say, Mobil can show some of the tests these lubricants go through and their results, too.

Maintaining oil thickness in a hot engine? This is measured in centistokes, which gauge the time it takes for oil to flow through a test channel, and fully synthetic Kirkland Signature 5W 30 rates at 11.6 centistokes at 212 degrees Fahrenheit, with the Mobil equivalent at 11.2. So the Kirkland oil flows a tiny bit slower, meaning it maintains its thickness under heat a bit better. Amount of zinc added to the oil for the anti-wear protection needed in older engines? Costco's oil has 870 parts per million versus Mobil's 868. Here, Kirkland has a (very) small advantage by providing more of the protective element. 

Those admittedly cherry-picked minor differences come with a huge price saving. Costco sells 12 quarts of Kirkland Signature for about $5 more than a six-pack of Mobil1.

Engine oil plays a number of vital roles in your car, so you have to be sure it's up to the task. Let's look at viscosity performance again. If oil is too viscous — that is, too thick — it may not flow through the engine quickly enough to lubricate the moving parts, leading to engine damage. Likewise, if the engine is too thin, the layer of lubrication between pieces of metal may be too thin as well, leading to the same problem. You need a "just right" formulation that's been properly tested to meet your car's needs.

Dirty oil can be another huge problem, as contaminants can lead to more metal-to-metal contact from lack of lubrication, plus the dirt can cause sludge buildups that prevent the oil from flowing where it should. It's usually the case that oil gets dirty and contaminated over time, which is why the simple act of maintaining your car is so important. But the American Petroleum Institute also has its own seal of approval for performance and quality to make sure new oil comes to you clean in the first place.