<PAGE>   1



                                                                  EXHIBIT 10 (d)

                                FOURTH AMENDMENT
                             TO EMPLOYMENT AGREEMENT

     This Fourth Amendment to Employment Agreement (the "Fourth Amendment"),
made and entered into effective as of April 14, 1998, by and between Quaker
State Corporation, a Delaware corporation (the "Corporation"), and Herbert M.
Baum, an individual currently residing in Irving, Texas (the "Executive").

                                   WITNESSETH:

     WHEREAS, the Executive is employed by the Corporation as its Chairman and
Chief Executive Officer under an Employment Agreement, dated as of August 1,
1994, as such agreement has previously been amended as of each of May 10, 1996,
January 1, 1997 and January 1, 1998 (such agreement, as amended, the "Employment
Agreement"); and

     WHEREAS, the parties desire by this Fourth Amendment to modify the terms of
the Employment Agreement in certain respects as hereinafter set forth;

     NOW, THEREFORE, in consideration of the mutual agreements contained herein,
the parties hereto hereby agree to amend the Employment Agreement, effective as
of the date hereof, as follows:

     1. Amendment to Paragraph 1(b). Paragraph 1(b) is hereby amended to
substitute the date "June 30, 1999" for the date "December 31, 1998" in the
first sentence thereof, and to substitute the dates "July 1, 2000, July 1, 2001
and July 1, 2002" for the dates "January 1, 1999, January 1, 2000 and January 1,
2001" in the second sentence thereof.

     2. Miscellaneous. Except as expressly amended hereby, all of the terms and
provisions of the Employment Agreement are hereby reaffirmed and remain in full
force and effect. The words "this Agreement," "hereof," "herein," "hereby" and
words of similar import when used in the Employment Agreement shall refer to the
Employment Agreement as amended hereby. This Fourth Amendment shall be governed
by and construed in accordance with the laws of the State of Delaware without
reference to principles of conflict of laws. The section and other headings
contained in this Fourth Amendment are for reference purposes and shall not
affect the meaning or interpretation of this Fourth Amendment. This Fourth
Amendment may be executed in any number of counterparts, each of which shall be
deemed to be an original and all of which together shall constitute one and the
same instrument.

     IN WITNESS WHEREOF, the Corporation and the Executive have executed this
Fourth Amendment as of the date first above written.

                                QUAKER STATE CORPORATION:

                                By:      /s/ Paul E. Konnney
                                    -------------------------------------
                                    Name: Paul E. Konney
                                    Title:  Senior Vice President, General 
                                             Counsel And Secretary

                                EXECUTIVE:

                                /s/ Herbert M. Baum
                                -----------------------------------------
                                Herbert M. Baum



                                       17