Can lower-viscosity motor oils improve fuel economy? We ask the experts

Most fleets have been approached by companies selling a magic bullet. “Just pour our product into your fuel tank or a crankcase,” the pitch goes, “and your trucks will see a fuel economy boost! Guaranteed!” Or there’s a slightly different variation: “Attach our device to your engines” – it’s usually a turbo-enhancement/air management contraption of some kind – “and you’ll see a fuel economy increase!”

The promised efficiency increases are usually in the 4 to 6 percent range. But like many products with bold claims, many of these magic bullets don’t work.

Such pitches leave most fleets weary and jaded over any new promises of increased fuel efficiency, but now there may be a magic potion that can be poured into the crankcase of diesel truck engines and boost that vehicle’s fuel economy by as much as 4 percent: Motor oil. Lower-viscosity motor oil, to be precise.

The concept behind this emerging theory is simple: Motor oil in an engine performs a variety of important functions, primarily providing lubrication to decrease friction between metal parts moving at high speeds. But the fact that the oil is present in the first place creates additional resistance inside the engine, causing it to work less efficiently as the combustion process takes place and power is transmitted to the transmission and drive wheels.

Higher-viscosity oils, the line of reasoning goes, offer greater lubrication benefits, but the tradeoff is that these thicker oils reduce efficiency, causing engines to work harder and burn more fuel. That’s why proponents of thinner low-viscosity motor oils say they will naturally allow engines to operate more efficiently, which will translate to improved highway fuel economy. But is there anything to it?

Just say yes

More oil companies are starting to say yes. “Just look at what has happened in the passenger car market over the past several years,” says John Demko, BP-Castrol’s technical service manager for the Americas. In the 1980s, engine manufacturers in that market began calling for lighter-viscosity oils. At the time, 10W/40 was the predominant grade; since then, passenger cars have transitioned through 10W/30 and then 5W/30 to 5W/20 in Honda and Ford engines and even 0W/20 today in certain Hondas and Toyotas.

“The main driver behind this trend has been better fuel economy,” Demko says. “As you lower viscosity, you are lowering internal viscous friction in the engine, and that helps contribute to better fuel economy.”

Demko admits that other factors also influence fuel economy, including vehicle weight, driver habits and engine design. “But the lubricant can also contribute positively to overall fuel economy,” he says. “That holds true on the heavy-duty diesel side of things, too.” As carbon dioxide emissions become a driving factor for the next generation of diesel engines, lower-viscosity oils are likely to be an important component in new engine designs because CO2 emissions are directly related to how much fuel is burned, Demko says. “With our recently introduced Elixion 5W-30, we have seen up to a 4 percent improvement in heavy-duty diesel applications,” he says.

Dan Arcy, OEM technical manager for Shell Global Solutions, says his company has demonstrated reductions in fuel consumption when using 10W/30 versus 15W/40, and that Volvo Trucks North America and other manufacturers specify lower-viscosity oil as a factory fill in their engines. Other manufacturers allow 10W/30 under strict operating conditions; Cummins will allow 10W/30 up to 100 degrees Fahrenheit. “With fuel prices going up, we are hearing more about this and getting more inquiries from our fleet customers,” Arcy says.

Many lower-viscosity oils are synthetic blends, and while the exact results may vary based on vehicle type, engine type, outside temperature, driving habits, driving conditions and current engine oil viscosity, most trucking fleets that change from a conventional mineral-based fluid to a high-performance synthetic lubricant will see improvements in fuel economy, says Marci Deuth, commercial vehicle lubricants product offer adviser, Americas, for ExxonMobil Lubricants and Specialties.

“Beyond possible fuel economy gains, fleets can generate a number of performance advantages and bottom-line benefits from using synthetic lubricants over conventional mineral oils,” Deuth says. These can include longer drain intervals since synthetic lubricants are more robust compared to conventional oils, extended equipment life, enhanced emissions system protection and improved cold weather starts.