1.  Field of the Invention
This invention concerns a filter system and its use to rejuvenate used lubricating oils. 
2.  Description of Related Art
The use of both single and two-stage filters for treating lubricating oils is known (see, for example, U.S. Pat.  Nos.  2,302,552; 3,390,778; 3,489,676; 3,749,247; and more recently European Patent Application number 88300090.3, having Publication number 0 275 148 and published July 20, 1988, the disclosures all of which are incorporated herein by reference). 
However, none of these publications disclose the particular filter system described hereinafter.