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The advanced technology in modern trucks rolling off the assembly line represents a new horizon for the work truck industry. This generation of heavy-duty engines is designed to produce fewer emissions and achieve increased fuel economy from the use of lower viscosity diesel engine oils. To meet their fuel economy targets, most heavy-duty engine builders have used API CJ-4/CK-4 10W-30 for years as their factory-fill with some now using FA-4 10W-30. In the larger service fill environment, CK-4 15W-40 is still the dominant viscosity grade representing nearly 75 percent of the heavy-duty engine oil sales in the US, but 10W-30 is the fastest growing grade—and for good reason.

In preparation for the new lower viscosity engine oils, field trials began nearly six years ago to validate the performance features. Phillips 66 accumulated more than 92 million miles of on-highway test data using API FA-4 oils and found that the FA-4 oils provided the same level of protection as the CK-4, but at a lower viscosity and with the benefit of increased fuel economy.

API CK-4 & FA-4

The previous category, API CJ-4, was in place for 10 years. During that time, engine builders made many significant changes to their designs leading to the need for an improved oil service category. The combination of higher power densities (smaller displacement engines providing more power), improved aerodynamics, and modern emissions systems resulted in higher engine temperatures, which increases the need for oils with enhanced thermal stability and oxidation control. Additionally, federal regulations continue to emphasize improved fuel economy for heavy-duty trucks, which resulted in the groundbreaking split categories—API CK-4 as a direct replacement of CJ-4, and the first-ever “F” category, FA-4, for fuel-efficient oils.

API CK-4 incorporates all the standard viscosity grades and is completely back-serviceable for all applications that successfully used the previous category. API FA-4 is an entirely new category driven exclusively by the industry’s need for enhanced fuel economy as contributed by the lubricant. These products are exclusively SAE 10W-30 and 5W-30 viscosities.


Perhaps the most confusing aspect of the new categories is the emergence of two different 10W-30 viscosity grades (CK-4 and FA-4). They are both the same Kinematic Viscosity (SAE 10W-30), so what is the difference? Simply put, the FA-4 10W-30 has lower internal fluid friction, which provides the fuel economy advantage since it takes less energy to pump it through the engine. This property, known as Absolute Viscosity (reported in centipoise [“cP”]), measures frictional drag within the lubricant itself using the High Temperature/High Shear (HTHS) test, which is designed to emulate the area between an engine’s crankshaft and the connecting rod journal bearings in a warmed-up engine. CK-4 requires a minimum HTHS of 3.5 cP (most are greater than 4.0) while FA-4 oils are in the range of 2.9–3.2 cP. It’s important to remember that FA-4 products must pass the same exact engine protection tests as CK-4 oils, but must do so while passing more rigorous HTHS standards. Holding FA-4 to the same performance standard ensures that there is no compromise in the level of protection provided.


A fleet with Class 7 and 8 trucks should reasonably expect that a move from 15W-40 to CK-4 10W-30 provides a fuel economy benefit of about 1 percent. FA-4 10W-30 should net about 1.5 percent savings, and FA-4 5W-30 showed 2 percent (up to 3 percent in Classes 3 to 6) better fuel efficiency in laboratory and field testing. Small savings can really add up.

For example: A fleet of 250 Class 8 trucks with 45,000-mile drain intervals averaging 100,000 miles per year at 6 mpg with $3.10 average diesel fuel cost could save up to $123,000 per year by switching from 15W-40 to CK-4 10W-30 (1 percent) or up to $181,000 with FA-4 10W-30 (1.5 percent). This savings projection can be adjusted up or down based on the size of your fleet. The Phillips 66 Lubricants Fuel Savings Calculator estimates fuel savings for fleets of different sizes and variables. And of course, more detailed analysis can be done on a local level.


While the annualized diesel fuel savings based on a 2 to 3 percent improvement should provide a significant financial temptation, the idea of jumping from a 15W-40 you’ve trusted for 20 years or more straight into a heavy-duty 5W-30 might be a little daunting for all but the most dedicated early adopters. In such cases, it’s perfectly reasonable to take a measured approach in the adoption of FA-4 engine oils.

Conservative fleet operators may consider a progression from CK-4 15W-40 to CK-4 10W-30 or FA-4 10W-30 simpler to implement. After a few years of experience with those products and with the gradual shift in fleet mix toward newer trucks, which are optimized to take full advantage of the lower viscosity oils available, a move to the full synthetic 5W-30 diesel engine oils will be easier to implement. If you’re operating Class 3 to 6 equipment, the leap to FA-4 5W-30 should be considered more quickly to take full advantage of the existing savings potential.


Phillips 66 recommends that you work with your local supplier to discuss opportunities for fuel economy improvement. These discussions should incorporate your typical duty cycles and maintenance goals to find the best balance for your specific equipment mix.


Tony Negri is the commercial product manager at Phillips 66. Find out more about the brand and access the Fuel Savings Calculator, visit


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