<PAGE>   1
                                                                     EXHIBIT 10j


                            [FINA, INC. LETTERHEAD]



                                 March 26, 1998



Mr. Neil A. Smoak
P.O. Box 601085
Dallas, TX 75360-1085

Dear Neil:

This letter, when executed by you in the space provided, will constitute an
Agreement between you and FINA, Inc. ("Company") concerning the termination of 
your employment and certain considerations, in addition to normal entitlements, 
relative to your retirement benefits. The terms of the Agreement are described
below:

1.   FINA agrees to pay a separation allowance in the amount of $242,500
     coincident with your separation from FINA on March 15, 1998. The allowance
     plus all remaining earned and accrued vacation will be paid in a lump sum
     payment, less required withholding.

2.   The Company acknowledges that you have earned a Vested Retirement Benefit
     under the FINA, Inc. Pension Plan ("Plan") based on your Benefit Service
     and Final Average Compensation (as those terms are defined in the Plan).
     The Company will provide you a total pension benefit as set forth on
     Exhibit A, and you may elect to start receiving this benefit on the first
     day of any month beginning with your Early Retirement Date ("ERD") of June
     1, 2001, through your Normal Retirement Date ("NRD") of June 1, 2011. If
     you elect to begin receiving a pension benefit prior to your NRD, it will
     be reduced, if applicable, to the amounts set forth on Exhibit A to reflect
     your age when you begin receiving your pension benefit. The pension
     benefits set forth on Exhibit A will be paid from the Plan to the extent of
     your Vested Retirement Benefit provided in accordance with the terms of the
     Plan, and any additional amount necessary to provide the full amount of
     your total pension benefit as set forth on Exhibit A will be paid from the
     general assets of the Company, either as part of your interest in the FINA
     Restoration Plan (discussed in Paragraph 3, below) or as a Supplemental
     Pension Benefit. You may elect to have your pension benefit, including the
     portion covered by the FINA Restoration Plan and the Company's supplemental
     contribution to your Vested Retirement Benefit, converted to a joint and
     survivor or certain and life option, in the manner provided in the Plan.

3.   The Company also acknowledges that you have earned certain benefits under
     the FINA, Inc. Restoration Plan ("Restoration Plan"). With respect to the
     Restoration Plan, you may elect any distribution option provided by the
     Restoration Plan. That portion of your interest in the Restoration Plan
     which replaces the Company's contribution to the FINA Capital Accumulation
     Plan may be taken immediately, and that portion of your interest in the
     Restoration Plan which replaces the accrual of additional benefits in the
     FINA, Inc. Pension Plan may be taken as a lump sum or an annuity beginning
     when you reach your ERD on June 1, 2001, but you may postpone receipt to
     the extent permitted by the terms of the Restoration Plan. (The benefits
     schedule set forth on Exhibit A assumes that your interest in the pension
     component of the Restoration Plan will be taken as a life annuity.) The
     benefits

<PAGE>   2
Mr. Neil Smoak
03/26/98
Page 2


     provided by the Restoration Plan will be paid from the general assets of
     the Company subject to any required withholding.

4.   The Company will waive the eligibility requirements for retiree medical
     benefits under the FINA, Inc. Comprehensive Medical Plan allowing you to
     participate as a retiree beginning March 16, 1998. The monthly cost of
     retiree medical care coverage for you and your eligible dependent will be
     determined in accordance with the provisions of the plan from time-to-time
     in force. Based on your years of service through your termination date, you
     will pay 52% of the cost of retiree medical coverage. Retiree medical
     premiums are generally adjusted on January 1 of each year.

5.   The Company agrees to provide you with financial consulting and federal and
     state income tax preparation services from the Ayco Company, L.P. for the
     calendar year 1998 (including the preparation of any 1998 tax returns filed
     in 1999).

6.   The Company agrees that you may have the Company's membership in the Dallas
     Petroleum Club which is registered in your name transferred to you upon
     payment by you of the Club's transfer or other similar fee prior to April
     30, 1998.

7.   The Company shall require any successor (whether direct or indirect, by
     purchase, merger, consolidation or otherwise) to all or substantially all
     the business and/or assets of the Company expressly to assume and agree to
     perform this Agreement in the same manner and to perform it as if no such
     succession had taken place. As used in this Agreement, "Company" shall mean
     the Company as hereinbefore defined and any successor to all or
     substantially all the business and/or assets of the Company that assumes,
     or is required by this covenant to assume, this Agreement by operation of
     law, or otherwise.

8.   The parties hereto acknowledge and agree that the statements contained
     herein are not merely recitations, but make and constitute valuable
     contractual considerations.

9.   This Agreement shall inure to the benefit of and be enforceable by your
     personal or legal representatives, executors, administrators, successors,
     heirs, distributees, devisees and legatees.

10.  No right or interest of yours under this Agreement may be assigned,
     transferred or alienated, in whole or in part, either directly or by
     operation of law, and no such right or interest shall be liable for or
     subject to any debt, obligation or liability of yours (other than a debt,
     obligation or liability of yours to the Company).

In consideration of, and as a precondition to this Agreement, except as to the
Vested Retirement Benefit provided in Paragraph 2 and the benefits under the
Restoration Plan described in Paragraph 3 on Page 1 above, the following
conditions will apply:

1.   Acting on your own behalf, and that of your heirs, executors, successors
     and assignees, you release and forever discharge FINA and any of its
     current and former affiliates, employees, agents, successors and assignees,
     from any and all claims under any State or Federal statute or common law
     related to your employment by or service to the Company, including, but not
     limited to, any claims for wrongful discharge, sex discrimination, race
     discrimination or age discrimination under the Age Discrimination in
     Employment Act, 29 U.S.C. Section 623, provided however, that no waiver
     contained in this paragraph


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Mr. Neil Smoak
03/26/98
Page 3



     shall limit or waive any right which you and your legal representatives,
     executors, administrators, successors, heirs, distributees, devisees and
     legatees may have to enforce this Agreement.

2.   You will use your best good faith efforts to return to the Company all
     documents (including copies) and other property of the Company that you
     may have received or had access to during the course of your employment
     which are the property of the Company and valuable to its ongoing business
     activities, and you acknowledge that you have not retained any such
     documents or copies.

3.   You acknowledge that you have been given 21 days to consider whether or
     not to sign this Agreement and that you had an opportunity to consult with
     any attorney of your choosing prior to signing it. You understand that you
     may revoke this Agreement at any time on or before the date which is seven
     (7) calendar days after the date of your signature on this Agreement and
     that the Agreement will not be effective or enforceable until the
     seven-day revocation period has expired. 

If this is your understanding of this Agreement, please execute and return the
copy of this Agreement, which is attached, to the undersigned.



                                             Sincerely,
                                             FINA, Inc.





                                             /s/ RON W. HADDOCK
                                             ---------------------------------
                                             Ron W. Haddock



ACCEPTED AND AGREED TO
this 26 day of March, 1998



/s/ NEIL A. SMOAK
---------------------------------
Neil A. Smoak
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Mr. Neil Smoak
3/26/98
Page 4


                                   EXHIBIT A

<TABLE>
<CAPTION>

                                          RETIREMENT PENSION BENEFIT(1)                   SUPPLEMENTAL PENSION(2)       TOTAL
                                                                                                  BENEFIT              PENSION  
                                                 MONTHLY BENEFIT                                                       BENEFITS
                                        ------------------------------------------------- ------------------------- -----------
Commencement                Actuarial       FINA, Inc.      FINA Restoration                 Reduction     Monthly      Monthly  
    Date         Age      Equivalent(3)  Pension Plan(4)      Plan(4)(5)        Combined     Factor(6)   Benefit(4)     Benefit
----------------------------------------------------------------------------------------- ------------------------- -----------
<S>             <C>       <C>            <C>              <C>                 <C>           <C>       <C>            <C>  
June 1, 2011     65         1.00000         $ 3,785.69        $ 2,930.73       $ 6,716.42      1.00    $     0.00    $ 6,716.42    
June 1, 2010     64         0.89270         $ 3,379.49        $ 2,616.26       $ 5,995.75      1.00    $   720.67    $ 6,716.42
June 1, 2009     63         0.79912         $ 3,025.22        $ 2,342.00       $ 5,367.22      1.00    $ 1,349.20    $ 6,716.42
June 1, 2008     62         0.71723         $ 2,715.21        $ 2,102.01       $ 4,817.22      1.00    $ 1,899.20    $ 6,716.42
June 1, 2007     61         0.64530         $ 2,442.91        $ 1,891.20       $ 4,334.11      1.00    $ 2,382.31    $ 6,716.42
June 1, 2006     60         0.58192         $ 2,202.97        $ 1,705.45       $ 3,908.42      1.00    $ 2,808.00    $ 6,716.42
June 1, 2005     59         0.52590         $ 1,990.90        $ 1,541.27       $ 3,532.17      0.95    $ 2,848.43    $ 6,380.60
June 1, 2004     58         0.47624         $ 1,802.90        $ 1,395.73       $ 3,198.63      0.90    $ 2,846.15    $ 6,044.78
June 1, 2003     57         0.43207         $ 1,635.68        $ 1,266.28       $ 2,901.96      0.85    $ 2,807.00    $ 5,708.96
June 1, 2002     56         0.39270         $ 1,486.64        $ 1,150.90       $ 2,637.54      0.80    $ 2,735.60    $ 5,373.14
June 1, 2001     55         0.35752         $ 1,353.46        $ 1,047.79       $ 2,401.25      0.75    $ 2,636.07    $ 5,037.32
</TABLE>

1.   Reflects benefits earned under the FINA, Inc. Pension Plan and the pension
     component of the FINA Restoration Plan.

2.   Reflects the supplemental pension benefits under the terms of this 
     agreement.

3.   Based on UP-1984 Unisex Mortality Table.

4.   Life annuity illustrated.  This monthly benefit may be converted to a joint
     & survivor or a certain and life option.

5.   This monthly benefit may also be converted to a lump sum payment.

6.   Early Retirement reduction factors from Section 5.2(b) of the FINA, Inc.
     Pension Plan.