Here is more info. from API. Please note that these pages may take a long time to load.

The actual license between Amsoil and API is here:
http://www.api.org/cgi-bin/eolcs_li.cgi?k=767

All API Certified CH-4 diesel oils:
http://www.api.org/cgi-bin/eolcs_li.cgi?c=CH%2D4


All API Certified CI-4 diesel oils:
http://www.api.org/cgi-bin/eolcs_li.cgi?c=CI%2D4

You can also note that there are some very, very small compaines that have API Certified motor oils. Some much smaller than Amsoil I'm sure. Here is a list of some of the smaller companies that did bother to get certified (I am not recommending any of them, but just posting some random names that appear to be very small compainies).

CAM2 OIL PRODUCTS, New Britain, PA 18901
CENTURY LUBRICANTS COMPANY, Kansas City, KS 66111
COASTAL UNILUBE, INC, Memphis, TN 38103
CONKLIN COMPANY INC, Shakopee, MN 55379
DAVID WEBER OIL COMPANY, Carlstadt, NJ 07072
DENNIS OIL COMPANY, Springfield, MO 65802
HYDROTEX, Carrollton, TX 75006
LUBRICANTS USA, Plano, TX 75075
LUBRICATING SPECIALITIES COMPANY, Pico Rivera, CA 90660
MCCOLLISTER & COMPANY, Council Bluffs, IA 51503
PINNACLE OIL INC, Indianapolis, IN 46268
ROYAL PURPLE, LTD., Porter, TX 77365
SCHAEFFER MANUFACTURING COMPANY, St. Louis, MO 63104
SCOT LUBRICANTS OF PENNSYLVANIA, Allentown, PA 18109
TOP OIL PRODUCTS COMPANY, Burlingame, CA 94010

And many others. Why did they bother to get their products API Certified and Amsoil didn't?