Another user (NoMo) made an interesting statement about Amsoil in another thread that caused me to do additional investigation. Thanks to NoMo for putting me on this trail. Please follow me on this logic because it's important.
Amsoil makes four motor oils they claim are for use in Diesel engines. Three are fully synthetic (30 weight, 5W-30, and 15W-40) and one is a synthetic blend (15W-40). Let's forget about the 30 weight oil and the synthetic blend oil for this conversation, and we'll concentrate on the fully synthetic oils that some here are using.
I will post verbatim from some sections of the 2002 Duramax Diesel Supplement page 6-18 which is titled "What Kind of Engine Oil to Use". Next to a graphic of the API service doughnut is the following statement: "This donught-shaped logo (symbol) is used on most oil containers to help you select the correct oil. It means that the oil has been certified by the American Petroleum Institute. You should look for this on the oil container, and use only those oils that display the logo." There is also a NOTICE that says "If you use oils that don't have one of these designations either CH-4 or CG-4, you can cause engine damage which is not covered by your warranty." You can read it yourself in your own manual for verification.
These are clear statements! Use only the proper API Certified oil or your warranty won't cover any damage.
I called the American Petroleum Institute to find out if Amsoil fully synthetic diesel oil is API approved (I talked to Kathy in the Motor Oils Division). I was told that Amsoil has been issued API License Number 0995 but that DOES NOT cover any of their multi-grade fully synthetic diesel oils. NoMo pointed out that the Amsoil does not carry the API starburst or donught-shaped logo.
Amsoil fully synthetic diesel oils are not API approved and are not approved for the use in your Duramax. API told me that all oils submitted for certification are tested by both their labs and an independent lab before a license for certification is granted. They would not tell me if Amsoil has submitted their oils and they failed, or if they have never submitted their oils for testing.
Amsoil claims that their synthetic diesel oils are "Recommended for Applications Requiring the Following Specifications:" and list various API ratings, but that is not the same a being API Certified, and not approved for the Duramax. They are telling you that you should beleive them that it meets API specifications, but we really don't know if it does or not. We do know it's not Certified as such. The only API Certified engine oils listed under the Amsoil license are the XL-7500 and PCO (synthetic blend) motor oils.
I asked about various fully synthetic diesel oils that ARE API Certified. They told me of many, but the one that stood out was issued API License Number 0022... Mobil Delvac 1. Treat your engine right and use the right oil.
I do not work for any engine oil manufacturer, and I do not sell any kind of motor oil. I did spend about 15 years in management in the automotive manufacturing industry. I have known some of the best automotive engineers in the world and I know that most of what they do is for a reason.
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