You could save as much as 1 percent on your fuel economy simply by switching from a 15W-40 engine oil to a 10W-30 engine oil, according to new research from Chevron Oronite.

A research paper, titled "The Lubricant Contribution to Improved Fuel Economy in Heavy Duty Diesel Engines," will be presented by Chevron Oronite engineers during the Society of Automotive Engineers' Commercial Vehicle Engineering Congress and Exhibition in Rosemont, Ill., Oct. 6-7.

The testing

Chevron installed equipment to conduct fuel economy testing using the Volvo D12D Fuel Economy Test, a lab-based test used in Europe, in its lab in Rotterdam in The Netherlands. This kind of testing allows researchers to precisely load the engine and run it under prescribed conditions the computer controls.

"Many of the OEMs have internal fuel economy tests, but most are not willing to release theirs because they are viewed as a competitive advantage," explains Gary Parsons, global OEM and industry liaison manager, Chevron Oronite.

The testing used a 15W-30 as a reference oil. The results were "weighted" with factors to represent hilly conditions or flat conditions.

In flat conditions, where obviously you would get the best fuel economy no matter what the oil, a 15W-40 oil performed nearly 0.8 percent worse than the base 15W-30. The 10W-30 performed nearly 0.2 percent better than the 15W-30. That's close to a 1 percent difference between the 15W-40 and the 10W-30.

"Based on our tests, we believe that SAE 10W-30 is a better choice than SAE 5W-40 for cost/performance," Parsons says. "Of course, all of the OEMs are concerned about any potential trade-off between engine durability and fuel economy. In other words, they don't want to recommend engine oil that may provide fuel economy benefits, but result in less durability. Much of our testing is focused on evaluating both aspects."

Why Lower Viscosity Saves Fuel

The oil pump in the engine basically sucks the oil out of the oil pan, and it pumps that oil and circulates it around in your engine to lubricate it - much like your heart pumps to circulate the blood in your body. The thicker that oil is, the more energy it takes to pump that oil, which uses fuel.

Just like people may take blood thinner to make it easier on their heart to pump it, it's the same with engine oil.

So why not go even lower? Why not a 5W-30?

The engine oils still has to be thick enough, to offer enough protection, to keep the parts from coming into contact with each other, to prevent wear. So you can't necessarily just keep going thinner and thinner without formulating the oil and additives to keep the durability up.

In a multigrade engine oil rating, the first number, like the 10W or 15W, is an indication of how thick the engine is under low temperature conditions. The second number, the 30 or 40, is how the engine behaves at about 210 degrees, at full operating temperature.

Most heavy-duty trucks and even midrange trucks operate much of the time at full operating temperature, Parsons explains. "So the difference between an XW-30 and a XW-40, you see a fuel economy different more related to the 30 vs. the 40 rather than the 10W vs. the 15W."

"A lot of people have 5W-40 synthetic heavy-duty motor oils out there, and they are used in low temperature conditions. Some people do see fuel economy benefits, particularly in pickup and delivery applications or other applications where the engine doesn't always reach operating temp, or there are a lot of cold starts."

While the 5W-30 performed even better in Chevron's test, at close to 0.4 percent over the base 15W-30 oil, vehicle operators may be hesitant to make such a dramatic change in viscosity grade from today's SAE 15W-40. In the case of 5W-40 weight oils, they require the use of synthetic base oils, raising the cost. 10W-30s do not, Parsons notes.

Durability

Many truckers in this country are skeptical of lower-viscosity engine oils, believing they won't offer enough protection.

However, lower-viscosity oils for heavy-duty engines are being adopted in other parts of the world. In Europe today the most popular grade for heavy-duty engine oil is 10W-40 and they are moving to 5W-30 in many cases. Parsons says, particularly in winter operations.

Part of the reason you can used lower-viscosity oils today than you could in the past is that the precision and the tolerances in the engine hardware itself is much better today. Parsons points out that today's engines are produced with high-tech machining practices that create surfaces with fewer microscopic peaks and valleys - more mirror-like.

The Future of Fuel Economy

Parsons also points out that there is a very strong likelihood that truck fuel economy standards will be coming in the next four to five years as part of government efforts to reduce greenhouse gas emissions.

"The last 10 or 15 years, the focus has been on reducing NOx and particulate emissions, which we've all done an incredible job on," Parsons says. "For 2010, those emissions are almost taken to zero. Now the focus will shift back to fuel economy."