Lowering Fuel Cost by Choosing the Right Engine Oil
Can motor oil affect a rig's fuel costs? The simple answer is "absolutely."
Market Drivers - Reducing Emissions & Increasing Miles Per Gallon
Stricter emission standards on new engines have been imposed by the EPA, targeting NOx compounds, particulates, and hydrocarbons. To meet those standards, engines were equipped with aftertreatment devices such as diesel particulate filters and diesel oxidation catalysts. Also compounding requirements on engine oil was the standard that dramatically increased the percent exhaust gas recirculated through the intake system. As engine designs changed to meet more stringent mandates, new engine oils had to be designed to keep those engines running smoothly.
When it comes to fuel economy, viscosity has a significant effect. Viscosity is a measure of resistance to flow. Oil is pumped and circulated around the engine. The higher the engine oil viscosity, the more energy required, hence, less fuel economy. The engine components must also move through the oil and ride along the lubricant film. Again, the higher the oil viscosity, the more energy required.
In other words, a thinner grade oil will lead to better fuel economy, assuming lubrication requirements are met.
So why not go even thinner on the motor oil for greater fuel savings? In some situations this has happened, but you can't go too thin on the oil unless it's been formulated with additives to maintain its durability in service. An oil grade that is too thin will not offer enough protection to keep moving parts out of contact with each other and prevent excess engine wear.
Let's take a multi-grade engine oil rating like 10W-30. A 10W oil is thinner than a 15W oil at low operating temperatures. The second number (30 in this case) indicates the oil's viscosity rating at 212°F (100°C) when the engine runs hot. However, most heavy-duty and mid-range trucks run at full operating temperature most of the time. So, the second number (the "hot" number) in the multi-grade designation is more important than the first, especially in warmer climates.
Depending on the situation, then, fleets may want to select their motor oil based on the "hot" viscosity rating. Thus, 10W-30 motor oil would be selected over a 15W-40 if fuel economy is the major issue. Truckers are sometimes skeptical about using thinner oils, but one of the reasons they can is that modern machining practices help OEMs build engines to tighter tolerances (and smoother surfaces) than in the past. Better tolerances means thinner oils can be used to coat moving parts and prevent wear.
It is critical to have a field engineer review your equipment before making a switch to a lower viscosity oil for fuel econmoy. Not all equipment and engines will tolerate this change. It is also critical to use a quality engine oil and maybe a synthetic, since the lubricant film will be thinner with lower viscosity oils.
So how much can you save just by changing your motor oil? By one research estimate, a switch from a 15W-40 oil to a 10W-30 oil saves 1% in fuel costs – a substantial savings over a year's time. This number is an estimate based on experiments designed to simulate highway driving conditions.