Save on inventory costs by using the same oil — CJ-4, CI-4/SN — in both gasoline- and diesel-powered vehicles. Photo: Romarti |
Save on inventory costs by using the same oil — CJ-4, CI-4/SN — in both gasoline- and diesel-powered vehicles. Photo: Romarti |

Before the 1990s, we used to change CD- and CE-classified diesel oils every 8,000 to 12,000 miles in Class 7 and 8 trucks, and more often for smaller ones. We did “dry” maintenance (chassis lubrication only) halfway between “wet” intervals.

Since then, we've had to adjust such routines to keep up with advances in engine oils and truck technology.

Most improvements in oil formulations have followed changes in emissions regulations. While refiners continually upgrade their products to remain competitive, the establishment of new standards is what drives real progress. The American Society for Testing and Materials (ASTM) and the American Petroleum Institute (API) continue to set higher performance standards for approving motor oils.

New oil classifications — CH-4, CI-4, CJ-4, and so on — reflect the most recent industry strategies for emissions. The “C” stands for compression ignition (diesel), the following letter indicates its place in the progression, and the number specifies use in four-stroke cycle engines. Oil industry convention is to use just the number.

When the U.S. EPA implemented its 2004 emission standard to reduce nitrogen oxides (NOx) in October 2002 — 15 months early — CI-4 oil was ready to control heavier soot loadings produced by exhaust gas recirculation (EGR) technology.

Shortly after its introduction, though, CI-4 was found to fall short of what was actually needed to meet the 2004 standard. CI-4 Plus took its place, with a modified set of requirements.

The next advancement, in 2007, was the introduction of diesel particulate filters to control particulate matter, consisting primarily of soot and metallic ash. CJ-4 was developed to minimize ash formation and suspend increasingly finer soot particles, thereby reducing filter plugging (see sidebar on next page).

For 2010, EPA regulations further reduced the levels of NOx and particulate matter allowable. Both CJ-4 and CI-4 Plus oils are considered current by API.

Some oils engineered primarily for diesel use can also be used in gasoline engines.

The API Service Symbol, or the “do-nut,” indicates the service appropriate for the oil. If it's marked “CJ-4, CI-4/ SN,” it can be used for all vehicles. That cuts inventory, at least for gasoline and medium-duty diesel engines that can both use 10W-30 oil. You can also use 15W-40 in gasoline engines, but fuel economy will suffer.

High-tech equipment

In addition to advanced formulations, new computer controls in trucks have extended the service life of oils.

According to Moore's Law, named for Intel founder Gordon Moore, the computing power of microchips doubles every 18 to 24 months. By a conservative estimate, computer capability increased 16 times between the 2002 start of the current series of emissions controls until the launch of EPA 2010 engines. The faster computers allow precision timing of multiple fuel injection pulses, which improves mileage and results in more complete combustion and less soot.

If you change oil and perform preventive maintenance according to manufacturers' recommendations, your routine need not change. But if you determine your oil drain schedule using oil analysis, and by following the American Trucking Associations' Technology and Maintenance Council Recommended Practices for determining proper engine oil change intervals (RP 334 for heavy-duty oil and RP 1403 for light- and medium-duty), you will need to repeat the exercise when you change API service classifications.

— Paul Abelson ( is a former director of the Technology and Maintenance Council (TMC) of the American Trucking Associations, a board member of Truck Writers of North America, and active in the Society of Automotive Engineers.


Several oils are compatible with exhaust after-treatment devices.

Specially formulated oils are designed to reduce exhaust pollutants and work with emission-controlling truck equipment. But fleet managers who invest in exhaust after-treatment systems may save money by using less expensive oil.

CJ-4 oil is formulated to extend the life of diesel particulate filters and to reduce catalyst poisoning and soot loadings. Lower-priced CI-4 Plus oils can be used in the engines, but they may shorten filter cleaning intervals and catalyst life.

One engine maker, Navistar, addressed EPA's 2010 regulations with Advanced EGR (exhaust gas recirculation), augmenting a proven technology with more precise computer control of fuel delivery. Even though more EGR was applied, better combustion limited soot formation, so no new oil was needed.

All other manufacturers adopted an exhaust after-treatment, called selective catalytic reduction (SCR). This nitrogen oxide-reducing process takes place outside the engine in the exhaust system, so no change in oil formulation is required. In fact, since this after-treatment allows reduced EGR, many fleets are using less costly CI-4 Plus in post-2010 engines.

Editor's note: For more on exhaust after-treatment technologies, see our January 2011 Fleet Management column by clicking here.