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.
Reading between the lines
As promising as this sounds, many people from all sectors of the trucking industry – including engine manufacturers, fleet managers and even oil companies – remain unconvinced that using lower-viscosity oils translates to improved fuel economy.
“The basic problem you get into is twofold,” says Mark Betner, Citgo’s product manager of heavy-duty lubricants. “Just how measurable is all of the fuel economy benefit, and is there a compromise with wear protection? The fuel efficiency benefit is not an easy thing to prove considering all the factors impacting fuel consumption.” Meanwhile, many believe higher viscosity provides better wear protection.
Citgo isn’t trying to steer fleets away from 15W/40 motor oils. “I think that will happen one day anyway, just as we once moved from straight-grade oils to multiviscosity oils,” says Betner, who points out that European trucking operations “don’t even argue this point anymore because their fuel has been at a higher price point for some time.”
The challenge with lower-viscosity lubricants and actual fuel economy performance not only is measuring the gain but also understanding the marketing of the savings claims. Typically the lower the size and weight of the vehicle, directionally the better the fuel economy savings that can be achieved – at least in laboratory testing conditions, says Len Badal, commercial sector manager for Chevron Lubricants Marketing. “At times this ‘hype’ in savings is advertised in the media, but it would not be the typical savings a Class 7 or 8 vehicle would actually see in real-world driving,” Badal says.
Betner also feels there are other advantages to lighter-viscosity oils beyond improved fuel economy numbers and says there are two ways fleets can look at the data when deciding for themselves. “If you just look at fuel efficiency, maintenance managers may say, ‘I don’t know if I can measure it, and synthetics cost a lot more, so therefore I’m going to stay with my cheaper 15W/40’ – which I understand,” he says.
Better in cold weather?
But Betner says Citgo has done a substantial amount of testing on lower-viscosity oils and has found additional benefits. “We’ve found they actually offer superior cold-weather fluidity and superior cold cranking compared to a 15W/40,” says Betner, pointing out that some engine manufacturers are concerned about cold-cranking and wear protection at startup in cold conditions.
In some cases, engine manufacturers will provide temperature ranges and the corresponding viscosity grade recommendations. Without block heaters, 15W/40 often is limited down to 15 or 20 degrees Fahrenheit for adequate pumpability. A cold box test designed to mimic real-world operation showed that 15W/40 oil took five times longer to pump at -22 degrees F compared to synthetic 5W-40 oil. “This also means longer starter and battery life,” Betner says. “If you run a fleet up north, that reason alone may be good enough to switch to 5W/40 oil.”
“I think that will happen one day anyway, just as we once moved from straight-grade oils to multiviscosity oils.”
– Mark Betner, Citgo’s product manager of heavy-duty lubricants
All engines have their own recommended viscosity ranges, and they’re all slightly different, so fleets must know where their engine manufacturers stand on this issue. “You must make sure it’s an acceptable grade of oil for the operating temperatures the truck works in,” Arcy says. Some engine makers currently don’t allow the use of lighter-grade motor oils, he says.
Another factor on moving to low-viscosity engine oils, especially with 5W/30 grades, is price. These oils typically are blended with full synthetics and cost more than traditional 15W/40s. “Customers will need to ensure they can measure the benefits to help offset the increased cost,” Badal says.
Betner believes fleets should do their own research on the matter using several trucks as part of a controlled test with the same drivers and routes for some degree of consistency and benchmarking. The citgohdlubes.com Website features a cost-calculator for users to input their fleet specifics to determine any savings from using low-viscosity motor oils.
Betner warns, however, that while most fleets are likely to see some positive results, not everybody will benefit. “But I think if they would look at the total performance capability of these oils from a standpoint of cold-temperature and startup wear protection, longer starter and battery life, fuel efficiency and possible inventory consolidation benefits, they would begin to look at them differently,” he says. “I truly believe the engine builders are going to start coming onboard with this in the future.”
