"Why are some AMSOIL Synthetic Motor Oils API licensed and some are not?
There are two primary reasons:
1. There are strict phosphorus additive level restrictions placed on API certified oils that would
minimize the full performance and extended oil drain capability of AMSOIL motor oils.
2. The API does NOT allow read-across approvals for synthetic motor oils, and therefore
certification costs become unrealistic.
Phosphorus Restrictions
Full API licensing, or certification, would impose strict phosphorous limitations on AMSOIL motor oils. This limitation is the main reason most AMSOIL motor oils are not API licensed...." (Copyright 2008)
http://www.syntheticwarehouse.com/api_licensing.htm
HAHAHA, Yeah... Go AMSOIL.... At least Mobil 1 can get API certs for their entire line.