In the US, I know these as the best:
www.analystsinc.com
Analysts, Inc.
800-222-0071
http://www.herguth.com/
Herguth Laboratories
101 Corporate Place
Vallejo, CA 94590
707 554 4611
Clark Laboratories, LLC
Jefferson Hills PA
(412) 387-1001 X 177
http://www.pltlab.com/
Petro Lubricants testing laboratories
Lafayette, NJ 07848
Phone 973-579-3448