My 2006 owners manual on page 373 states to use oil conforming to API service CI-4+. No where does it say I can only use oil that has been tested by the API and has the API logo. The Amsoil diesel oil I use in my engine meets or exceeds the API spec for CI-4+, Cummins spec and the specs for a few other brand of engines and Amsoil carries a 2 million dollar insurance policy for there oil.

As I understand it it cost a lot of $$$$ to have oil tested by the API. If Amsoil was to make any changes to the oil they have to pay to have the oil with changes tested before the improved oil can use the API logo, so by not paying to use the API logo it saves them and me money.