AMSOIL Signature Series 15W-40 Max-Duty Synthetic Diesel OilA reader asks: I’ve got a 2003 Dodge Ram 2500 with the 5.9L Cummins engine. Should I use the older generation of CI-4+ diesel oils? Are the newer API CK-4 diesel oils good for this engine? My understanding is that these engines have flat tappet cams and I hear that the newer diesel oils do not have enough zinc to protect them.

Our answer: The short answer is that (for your older engine), virtually any of our diesel motor oils will be highly effective. The long answer may surprise you.

There is a common misconception that the newest diesel motor oils meeting the latest API CK-4 specification are somehow weaker or less robust than previous generations of diesel oils. This is patently false.

In fact, API CK-4 diesel oils are considerably more sophisticated and capable than diesel oils meeting previous specifications such API CI-4+. Older diesel oil formulas meeting API CI-4+ are weaker in terms of protective capabilities.

The introduction of the CK-4 spec was a tremendous challenge for motor oil makers. Diesel engines and their emissions components have become very sophisticated in recent years. The CK-4 requirements required oil makers to ramp up their game to meet much higher standards. Here are some of the upgrades of the API CK-4 spec over the previous CJ-4 spec.

  • Increase in high temperature capabilities
  • Improvement of piston and bore wear protection
  • Better deposit control
  • Resistance to degradation of hot and cold weather properties
  • Improved shear stability
  • Enhanced aeration resistance
  • Resistance to catalyst and particulate filter fouling
  • Improved oxidation resistance

What About AMSOIL Heavy Duty Series Vs. Older Diesel Oils?

AMSOIL Heavy Duty Diesel Series is a less expensive API CK-4 option to the flagship Signature Series Max-Duty Diesel Series. Heavy Duty Series was created as a budget-friendly option for fleets, the oil change installer market and those looking for a less expensive synthetic product.

We asked AMSOIL Technical Services how the API CK-4 AMSOIL Heavy Duty 15W-40 might compare to an older generation product like the API CI-4+ “AME” AMSOIL Heavy Duty Diesel and Marine 15W-40. Their answer was “It’s not even close”. While the Heavy-Duty Diesel And Marine 15W-40 is a very robust and competent product, development stopped when the API CI-4+ specification was replaced in 2007. The ball has been moved way down the field since then. So even the less expensive API CK-4 diesel oils are a step up over the previous generation.

Signature Series Max-Duty is considerably more advanced. It offers the most advanced synthetic base oil and additive technology available. As a result, Signature Series Max-Duty is an upgrade over Heavy-Duty Series in terms of wear control and service life.

What About Flat Tappet Cams?

Flat tappet cams are relatively common to many diesel engine types. The biggest concern with flat tappet cams is wear protection. Specifically, does the motor oil contain enough ZDDP (Zinc Dialkyl Dithio Phosphate) and phosphorus?

Flat tappets in diesel engines are less of a concern than having flat tappet cams in something like a gasoline-powered street rod motor. Street rods and drag racers run at much higher RPM and tend to have much heavier valve springs compared to stock diesel engines. Also, modern passenger car/gasoline motor oils have much lower zinc and phosphorous limits. These levels are insufficient for flat tappet wear protection. So owners of gasoline engines with flat tappet camshafts have to be very mindful of their motor oil choice.

This is not a concern with diesel engine oil. Diesel motor oils are designed for all diesel engine types. Contrary to popular belief, the newest API CK-4 spec has not limited zinc and phosphorous to dangerously low levels with 40-weight viscosities. This would include 0W-40, 5W-40 and 15W-40, etc.

The API CK-4 AMSOIL Signature Series Max-Duty 5W-40 and 15W-40 has zinc levels over 1300 and phosphorus levels above 1200 ppm. Similarly, the AMSOIL Heavy- Duty 5W-40 and 15W-40 has zinc over 1100 ppm and phosphorus over 1000 ppm.

To contrast, the older AMSOIL Heavy Duty Diesel and Marine 15W-40 (API CI-4+) has 1377 PPM ZDDP and 1267 ppm phosphorus. This doesn’t necessarily mean that it will protect flat tappet engines better than the API CK-4 oils. The zinc and phosphorus level comparisons are a moot point. The newer API CK-4 chemistry is miles ahead of the older API CI-4+ formula. The newer CK-4 options will provide considerably better wear protection.


To sum up, do not be concerned about using the latest API CK-4 diesel motor oils in older engines. They are very much backward compatible with older engine designs and are a giant step forward in terms of wear control.