Kirkland vs SuperTech: What It Means for 5W-40 Buyers

Most engines calling for 5W-40 are diesels or high-performance gas engines, and the shopper standing in the oil aisle is usually deciding between a well-known name brand and a cheaper store-brand option sitting right next to it. No dedicated 5W-40 head-to-head exists in the testing corpus behind this guide, so this is an honest gap: the exact viscosity was not tested. What was tested, in a real two-brand comparison, is 5W-30 synthetic oil from a store brand against a pricier competitor, and the store-brand-versus-name-brand question at the heart of that test is exactly the question most 5W-40 shoppers are actually asking.

What the testing showed

The numbers below come from Project Farm's synthetic motor oil comparison, a 2-brand test of Kirkland Signature and SuperTech, both full synthetic 5W-30 oils. This is a 5W-30 test, not 5W-40, and the guide is upfront about that difference, but the store-brand-versus-competitor comparison speaks directly to the value question that applies across viscosity grades.

Kirkland Signature Full Synthetic, at 24.99 dollars for 10 quarts (about 12.50 dollars per 5-quart equivalent), claims advanced wear protection, control of thermal breakdown, Dexos 1 Generation 2 approval, and protection against low-speed pre-ignition in modern turbocharged, direct-injected engines. It meets SAE 5W-30 API SN Plus/SN/SL and ILSAC GF-5/GF-4 or previous standards, and is blended by Warren Distribution in Omaha, Nebraska.

SuperTech Full Synthetic, at roughly 35 dollars for the equivalent 10 quarts at Walmart, roughly 10 dollars more than Kirkland for the same volume, self-rates itself at 3 stars for reduced engine wear, 3 stars for combatting sludge and deposits, and 3 stars for extreme temperature protection, and meets API SN Plus/SN/SM/SL and ILSAC GF-5/GF-4 standards.

The evaporation test, the one hard number available

An evaporation test at 420F over 2 hours, measuring weight loss as a proxy for how much oil breaks down and burns off under sustained high heat, showed the two oils performing almost identically. SuperTech started at 411 grams and ended at 403.33 grams, a loss of 7.67 grams. That was, by a razor-thin margin, "the smaller loss of the two brands by 0.02 grams," meaning Kirkland's evaporation loss was essentially the same, differing by an amount too small to represent a meaningful real-world difference.

That result is the core finding of this test: two oils priced 10 dollars apart per 10-quart jug performed within a rounding error of each other on the one hard, measured test available.

How to read this for your own purchase

Since 5W-40 itself was not covered by any cited test, apply the value lesson this near-identical 5W-30 result demonstrates directly to your own 5W-40 purchase.

Check for the certifications your engine actually requires, then buy on price. Both oils in this test met the same core API and ILSAC standards despite a meaningful price gap, and their one measured performance difference was negligible. If a 5W-40 store brand carries the same API/ACEA certifications your owner's manual calls for, the certification, not the brand name, is what protects your warranty and your engine.

Don't assume a higher price buys better heat resistance. The pricier SuperTech in this test barely edged out the cheaper Kirkland on evaporation loss, by an amount unlikely to matter over a normal oil change interval, which undercuts the common assumption that spending more automatically buys more thermal protection.

Consider your climate and driving pattern before assuming a thicker 40-weight is automatically the safer choice. A 5W-40 oil holds a higher viscosity at operating temperature than 5W-30, which suits engines under heavier load, higher mileage, or sustained high heat, but an engine spec'd for 5W-30 does not automatically benefit from switching to 40-weight, since the wrong high-temperature viscosity can affect oil flow to critical components just as much as using an oil that is too thin.

A few universal rules this test supports for any synthetic oil purchase, 5W-40 included:

• Match the viscosity grade to your engine's manual, not to whichever oil is cheapest that day. 5W-40 and 5W-30 are not interchangeable; the second number reflects a different high-temperature viscosity behavior suited to different engine designs, particularly diesels and some European gas engines.
• A store-brand synthetic oil meeting the same certification standards as a name brand is a legitimate, tested-equivalent choice, based on how closely these two oils performed on the one hard measurement in this comparison.
• Look past star ratings printed on the bottle. SuperTech's self-assigned 3-star ratings across its claims did not translate into a meaningfully better measured result than Kirkland's unrated but equally certified formula.

Factor in your vehicle's actual mileage and engine wear when weighing viscosity. Higher-mileage engines with more internal clearance often benefit from the slightly thicker high-temperature protection a 40-weight oil provides compared to a 30-weight, one reason 5W-40 is common in older diesel and performance engine specifications, a consideration this cited 5W-30 test could not address directly since it did not compare across viscosity grades.

Browse more engine oils and fluids measured against their own claims on the engine oil and fluids hub.