Is 5W-20 Oil Worth It? A Four-Viscosity Bench Test

5W-20 sits in an odd spot on the oil shelf. It is thinner than the 5W-30 that older cars still call for, but thicker than the 0W-20 and 0W-16 grades that newer engines are increasingly built around. If your manual specifies 5W-20, you are basically being told your engine needs a middle-ground oil, and it is fair to ask what that middle ground actually buys you over the grades on either side of it.

A hands-on lab test that ran four viscosities of the same oil line side by side gives a genuinely useful answer, because it isolates viscosity as the only variable. Same brand, same base formulation, four different thicknesses, tested on identical rigs.

What the testing showed

The numbers below come from Project Farm, who tested four viscosities of Pennzoil Platinum full synthetic oil, 0W-16, 0W-20, 5W-20, and 5W-30, through the same battery of tests. You can watch the full comparison on the motor oil viscosity test. This is a single-brand viscosity study, not a multi-brand contest, and the video explicitly declined to name an overall winner, since the whole point was to measure the tradeoffs of thickness rather than crown a champion.

Where 5W-20 landed across the tests

• Room-temperature flow race (all four oils): 5W-20 finished 3rd, very close behind the 0W-20, in both the fresh and heat-exposed runs.
• Thermal breakdown/evaporation (200g heated to 410F for 2 hours): 5W-20 lost 3.92 g, the second-best (second-lowest) evaporative result of the four, just behind the thickest oil, the 5W-30.
• Cold flow at negative 40F, fresh oil: 1 minute 57 seconds, just behind the 0W-20's 1:54, both needing about a 45-second warm-up before they began moving at all.
• Wear scar (lubricity) test against 0W-20: 5W-20's wear scar was about 7 percent smaller than the 0W-20's, meaning slightly better wear protection in that direct pairing. The closing summary described 5W-20 as performing "almost as well" as the thicker, slower-flowing 5W-30 on wear protection.

That last point is the interesting one. Across the whole four-way comparison, 5W-20 came out looking like a genuine middle ground rather than a compromise: cold flow close to the thinner grades, wear protection close to the thickest grade. The video itself never frames it this explicitly, but the numbers put 5W-20 in the best balance-of-protection-and-flow position of the four oils tested.

A separate multi-brand motor oil test that included several 5W-20 and 0W-20-rated products showed how much brand-level variation exists within any single viscosity class. Across six full synthetic oils, evaporative loss ranged from 6.17 g to 7.72 g of a 200 g sample, roughly a 25 percent spread, and the cheapest oil in that test ($12 a quart) won both room-temperature flow races outright. Viscosity explains part of an oil's behavior; brand and additive package explain a lot of the rest.

How to read this for your own purchase

The viscosity test's bottom line matters more than any single number in it: it refused to declare a winner and told viewers to simply run whatever grade their manufacturer specifies. That advice holds regardless of how good 5W-20 looks in this particular comparison, because engine tolerances and oil pump design are built around a specific viscosity target, not a lab scoreboard.

If your manual calls for 5W-20, the data is reassuring. It landed close to the thinner grades on cold flow and close to the thickest grade on wear protection and evaporation resistance, without the worst result in any single category.

If you are deciding between 5W-20 and 5W-30 for an older engine that technically allows either, note that 5W-30 had the best evaporation resistance and wear protection of the four grades tested, but was clearly the slowest to flow, both at room temperature and especially in the cold. That is a real tradeoff, not a rounding error.

If you are shopping between brands within the 5W-20 grade, viscosity testing will not help you there, since it holds viscosity constant. Look instead at the brand-level spread the multi-brand oil test exposed: nearly a 25 percent difference in evaporative loss among full synthetic oils marketed almost identically.

It is also worth separating two different decisions that shoppers often blend together: which viscosity to run, and which brand to buy within that viscosity. The viscosity test settles the first question reasonably well for anyone choosing between adjacent grades. It has nothing to say about the second question, because it held brand constant on purpose. If you already know your engine calls for 5W-20, your remaining decision is entirely a brand-and-formulation question, and that is where the multi-brand motor oil test becomes the more useful reference, since it showed real, measurable gaps in evaporation resistance and additive package strength between full synthetic oils that all carry a 5W-20 or comparable rating on the label.

A few rules the numbers support no matter which viscosity you land on:

• Match the cap, not the lab. Both tests cited here ultimately point back to the manufacturer's specified viscosity as the right call.
• A middle-grade oil is not automatically a compromise. 5W-20 traded almost nothing on cold flow to gain most of the wear protection of the thicker 5W-30 in this test.
• Brand matters as much as grade. The spread between full synthetic brands in evaporation and lab additive content was larger than the spread between adjacent viscosity grades.

Want to compare full synthetic oils by brand instead of by grade? Browse the rest of the engine oil and fluids tests for the complete lineup.