Exploring The Pros and Cons of Ultra-Low Viscosity Oils: Are They Just For Emissions?

Ultra-low viscosity oils are becoming the norm for modern engines, especially smaller units prioritizing efficiency. Unfortunately, the trend has resulted in conflicting opinions about the actual usefulness of thinner oils. Manufacturers like them because they help them pass emissions and newer engines have been designed with tighter tolerances. But some enthusiasts and mechanics are skeptical that they’re actually better in terms of protecting an engine over the long haul.

While the viscosity of any fluid is always relative to what it’s being compared to, we’re specifically discussing some of the newer “ultra-low” viscosity oils — like 0w-16 or 0w-8 — manufacturers started recommending a few years ago. Compared to something like the common 5W-30, these are incredibly thin oils. However, 5W-30 would be considered thin when stacked against something like 20W-50.

Lower viscosity oils tend to flow better when cold, meaning they should offer better engine protection upon startup. They’re also prone toward offering improved fuel economy and lower emissions, as studied by SAE International. Oil pumps may have an easier time with them and they should flow more freely than something thicker, too.

But there are concerns that they offer reduced protection at higher engine temperatures and are more prone toward evaporation. Thinner oils likewise have a tendency to yield lower oil pressures than their more-viscous counterparts, potentially resulting in less-comprehensive lubrication and premature wear. The last concern revolves around claims that ultra-low viscosity oils need to be changed more frequently due to allegations that they’re poised to see more thermal breakdown.

By contrast, thicker oils typically offer less protection upon startup and provide improved protection at higher engine temperatures. They can also offer too much oil pressure for certain applications or insufficient flow on motors that aren’t specified for them. But they’re likewise prone toward making an engine run a little smoother (and quieter), particularly if it’s an older motor with a surplus of miles, and can reduce oil consumption as seals begin to fail. There is some solid data that they can produce a longer lasting film on engine parts, potentially helping to reduce wear over time.

We also need to talk briefly about additives, which vary dramatically between oils (including those of identical viscosity). Additives can include detergents designed to break down engine sludge, elements to keep oil from oxidizing and breaking down prematurely from contaminants (e.g. fuels), corrosion inhibitors that are supposed to help components from rusting, pour-point dispersants to help maintain reliable flow at low temperatures, viscosity modifiers that do the same thing at variable temperatures, anti-wear additives that are supposed to coat components in a protective film, and more.

Those are immensely important in determining how an oil functions inside of an engine. But we’ll have to gloss over them for the remainder of this article because there are just too many variables to take into account. There are even tests suggesting that additives actually promote engine wear, as outlined by Motor1.

We’re going to try and keep things basic and remind readers that the rule has historically been that it’s okay to run with a slightly thicker oil than a manufacturer recommends. But going thinner has typically been a bad idea unless you’re running your vehicle in an extremely cold environment where a thicker oil may hamper startup. That’s not a recommendation to swap to a different viscosity, however. You’ll need to read your manual and make those kinds of decisions for yourself, as going too thick or thin is still bad (potentially even catastrophic) for your motor depending on its own tolerances.

If you’re curious to see the core differences between thicker and thinner oils, there are few interesting experiments. Project Farm has done loads of testing in order to showcase the different properties of specific oil types and has an excellent video pertaining to varying oil viscosity. It’s a solid visual representation of all the concepts we’ve discussed so far. The takeaways were basically that (assuming all other variables are equal) lower viscosity oils should flow better at colder temperatures whereas thicker oils offer better protection against wear even after being subjected to intense heat and higher average pressures.

Engine Masters also tackled the topic. But their testing was focused specifically upon oil performance in larger and older high-output engines (specifically a Chevy 454 Big Block V8). Regardless, the associated discussions cover a lot of ground and remain useful for our purposes and reinforce most of our previous statements.

The really tricky bit has been sourcing good data on smaller, newer engines that manufacturers have specified need ultra-low viscosity oils. We’ve seen models using the same engine on different markets using varying oil thicknesses, presumably to appease local emission regulations. But that doesn’t necessarily mean that they’re worse/better for your engine.

Engineering Explained released an interesting video pertaining to modern engines and claimed that “engine wear is not usually a limiting factor” on modern engines, adding that manufacturers have effectively figured out how to make internal components last and are now using oils to tackle efficiency. But that statement would have held a little more weight if the video wasn’t sponsored by Mobil1 — which produces quality oils but also has an impetus to sell the latest blends to stay in businesses.

Does that mean we should discount the information provided by Engineering Explained? Absolutely not. The video still addresses some of the concerns pertaining to varying needs of different types of engines and indirectly suggests that thinner oils may even offer reduced protection when a piston is at top or bottom dead center. But it still ends by suggesting that you should never use anything other than the manufacturer specified fluids and trying to reinforce the point that ultra-low viscosity oils are fine.

The above examples provide additional context and visual representations. But none of them really address the real world concerns of drivers who now own newer vehicles specifying exceptionally thin oils. Sadly, there’s not a lot of good data because neither the relevant engines or the oils themselves didn’t really start appearing until the 2010s. We need specific, citable examples with irrefutable data that sadly doesn’t yet exist in great quantities.

There has been anecdotal evidence and an abundance of intelligent speculation, however. When covering the engine fires pertaining to the first-generation Toyota GR Corolla, there wasn’t much of a consensus on what caused the issue. While common enough to garner public attention due the vehicle’s likelihood of being tracked and obsessed over by owners, the failures were also so rare that there weren’t many people doing comprehensive mechanical autopsies to determine what actually happened.

Now that some time has passed, there are a few working theories, most of which involve irregular oil pressure values preceding the fires. Plenty of these blame aftermarket components being fitted to the car. But several allege that the stock internals may not be up to the task and that there’s a greater potential for thinner oils to blow by the piston rings. In most cases the affected cars showcase a hole where a connecting rod shot through the engine block and excess oil residue inside the turbo charger and exhaust pipe.

A lot of attention has been given to the piston rings and connecting rods. But nearly as much has been paid to the oil line leading to the turbo charger, something which I believe Toyota beefed up on the later models. There have been claims that the 0w-20 oil that Toyota calls for simply isn’t sufficient for high-performance applications. PIRAS Motorsport, which specializes in the GR Corolla and Yaris (both of which use the 1.6-liter G16E-GTS pictured below), has asserted that it’s better to run a much thicker 5w-30 or 5w-40 — especially if someone plans on taking their vehicle to the track or modifying the powertrain.

The theorized reason for this is that the thinner oils, combined with a presumed lackluster seal around the piston rings, could allow enough blow-by for the oil to mix with fuel and ignite within the direct ignition chamber when the cylinder isn’t on the combustion portion of its cycle. It’s certainly a plausible explanation. But we also need to remember that the G16E-GTS is an exceptionally high strung motor with uneven firing intervals.

While racers have had good success with putting thicker oils into track cars designed to endure higher engine temperatures forever and I’ve had decent luck putting slightly thicker than specified oils into automobiles that are well past their prime, I’m not totally convinced that that running with a higher-viscosity oil is going to make all the difference on modern vehicles like the GR siblings.

But I could say the same when manufacturers claim they’re not using ultra-low viscosity oils specifically to help pass emissions. The truth, as usually, presumably resides somewhere in the middle. I don’t personally spend enough time inside modern engine bays to have a definitive opinion on the matter. However, that has not prevented me from obsessing over any topic that pertains to premature engine wear and it would be nice to have a definitive answer before I go out and purchase a vehicle with 0w-8 written on the oil cap.

Seeking advice from professional mechanics has led to mixed results. Many of the professional gear heads I spoke to suggested that modern engines, particularly hybrids, needed thinner oils so that the fluid can be sent to the necessary components as they cycle on and off. Others stipulated that the lubrication channels on many newer engines may be too small for thicker oils to perform adequately. Concerns about heat and proper lubrication were pretty common. But most likewise agreed that the ideal solution would be to run as viscous an oil as you can safely get away with to minimize wear over an engine’s lifespan — noting that you should probably just stick with the manufacturer-recommended oil to avoid complicating any potential warranty claims.

It got my wheels turning mentally, but didn’t really satisfy me. My impromptu mechanic survey likewise failed to address concerns I had with newer user manuals recommending something like 0w-16 and then suggesting using a higher viscosity lubricant if the vehicle was going to be “operated at high speeds, or under extreme load conditions.” Nor did it make me feel better about the exact same powertrains sometimes having different manufacturer suggested oils depending upon which country the car was sold.

Feel free to throw in your two cents on the issue. Because I’ve been exploring it for the last few months and still feel like I’m only scratching the surface like an inadequately lubricated piston scratching the cylinder wall.

[Images: Jacktamrong/Shutterstock; Konstantin Shadrin/Shutterstock; Toyota; DedMityay/Shutterstock]

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