Currently there are a handful of Re-refining companies in the Unites States that process used oils to manufacture clean base oils that are suitable for blending into motor oil or other lube oil products. Generally the Re-refineries also operate their own collection facilities so they compete with collectors for access to used oils from gatherers. Collectors sell most of their used oil as a low grade black fuel oil.

Re-refineries are distinguished by 2 factors:

They utilize the same technology used by partial Re-refineries plus they use hydrogenation technology or other regeneration technologies to remove the sulfur, improve stability, and to control the color and odor characteristics of the used oils. 

They produce a base oil that can be used in the production of motor oils and industrial oils comparable to virgin base oils. The lube oils used to blend motor oil meet API certification and International Lubricant Standardization and Approval Committee (ILSAC) standards. These can be advertised and displayed on product packaging by showing API's motor oil quality marks----the API Service Symbol "Donut" and Certification Mark "Starburst" which help consumers identify quality motor oils for their gasoline and diesel-powered vehicles.
Source: U.S. Department of Energy