Having only been widely available on the market for the last four years or so and coming in at an oddly low price, it's easy to be suspicious of its performance. But given Costco's reputation for sporting high-quality brands without the fancy marketing of its competitors, it shouldn't come as a surprise that this efficient and economical choice has been mostly well-received by users.
Available in SAE 5W-30, SAE 5W-20, and SAE OW-20 viscosities, the synthetic oil claims to provide effective protection to your engine while also controlling thermal breakdown. It's been able to meet oil standards set by API SN Plus and ILSAC GF-5 and the viscosity specifications of the Society of Automotive Engineers when set under varying temperatures, meaning it is designed to allow for maximum fuel efficiency and engine protection.
Ultimately, as long as you don't skip on regular oil maintenance, this brand is great for a wide range of vehicles from older cars to gas trucks.
While Costco is the primary retailer of Kirkland Signature Synthetic Motor Oil, it is not its manufacturer. Like all Kirkland products, the motor oil is instead farmed out to a separate manufacturer before being put on store shelves. That distinction belongs to Warren Oil. While Warren carries a handful of its own retail brands such as Gold Brand and Lubriguard, it is most well known for providing a wide range of greases, lubricants, and chemicals to various retailers.
When looking at Kirkland Motor Oil, the brands it is most comparable to are Walmart's Super Tech and Amazon Basics Synthetic Motor Oil, both of which are also manufactured by Warren Oil. These oils meet similar specifications and standards from across the industry while also performing well in a variety of tests.