In North America, 15W-40 is the predominant viscosity grade for heavy-duty diesel engine oil. While other options exist, such as 5W-40 and 10W-30, many consumers are reluctant to change from the time-proven 15W-40 products. But there are compelling reasons to consider either a 5W-40 or a 10W-30, depending upon your application and climate.
Oil technology, like everything else, has progressed over the past several years. Yet, there is a reluctance to change. "The average customer has an appetite for higher viscosity," says Mark Betner, Citgo Petroleum heavy-duty lubricant product manager. "Many people have what I call 'Viscosity Fear Syndrome'. When you talk about low-viscosity oils, they envision higher cam wear, higher bearing wear. In reality, a diesel engine operates in a lubrication mode that basically maintains full film lubrication as compared to a gasoline engine. Higher viscosity oils actually result in robbing more power out of the engine."
Frictional losses in an internal combustion engine can be significant. "Up to 30% of the power that is generated by a diesel is just running itself," Betner points out. "The rest goes to mechanical work energy. When you are turning 1,600 rpm, anything that increases friction will draw more energy out of that engine."
But perceptions die hard. Betner recalls many years ago when multi-grade 15W-40 was introduced. He had a customer who stubbornly refused to switch from a straight 30-weight oil. "The customer told me to go back and tell the engineers that a 15W-40 will never work in a diesel engine. That was based on past experience, lack of information and lack of education," he says.
Customers need to feel confident before they are willing to change oil grades. "The largest obstacles are going to be acceptance, understanding and feeling comfortable with it," says Dan Arcy, OEM technical manager for Shell Lubricants. "We have all been using 15W-40 for 30 years now. Any kind of change takes time. That will slow some of the transition."
"It's a combination of education and comfort zones," agrees Betner. "The reason why 15W-40 has such a strong hold on the diesel market is that's what the engine manufacturers have primarily recommended."
Though there are a few manufacturers who do limit the viscosity to a specific grade, most manufacturers allow a range of viscosities, depending on the ambient air temperature. So, while it's vital to adhere to the manufacturer's recommendations, you almost have to look at it manufacturer by manufacturer and engine by engine to make sure a 10W-30 or 5W-40 is acceptable.
The temperature range of the climate in which you operate should play a large role in your oil selection.
If you study the viscosity charts from the manufacturers (remembering that the first number represents cold weather performance and the second represents high temperature performance), 5W-40 oils often offer the widest ambient temperature operating range, says Betner. For instance, a published Caterpillar viscosity chart rates a 15W-40 from +15° F to 122° F ambient. A 5W-40 is rated from -22° F to 122° F ambient. That gives you an additional 37° F lower ambient for cold starting.
As such, the tried and proven 15W-40 simply isn't the best solution in some cases. "A 15W-40 will not flow properly below 15° F," says Betner. "Many people don't realize it is not just the oil flowing into the rod and main bearings or creating proper film. There is also a battery and a starter. If you don't artificially heat the engine, or use outside energy to heat it, you are going to crank that engine and the battery has to supply amperage to the starter, which in turn has to convert electrical into mechanical energy."
The thicker oil is going to require the battery to deliver more amperage every time you cold crank it. "The starter is going to have to work harder," says Betner. "It is going to heat up faster. Every time you draw a battery down lower because you are requiring more from it, and every time you overheat a starter, you are going to shorten the life of those two components."