<PAGE>

                                                                   EXHIBIT 10.15



                       UNOCAL SUPPLEMENTAL SAVINGS PLAN

                          (Amended December 5, 2000)

The Unocal Supplemental Savings Plan (the "Plan") is amended and restated as of
December 5, 2000.  The Plan is maintained by the Company primarily for the
purpose of providing deferred compensation for a select group of management or
highly compensated employees by permitting certain of such employees who are
eligible to participate in the Unocal Savings Plan to receive contributions
hereunder if they are subject to limitations on contributions resulting from the
operation of Section 401(a)(17) of the Internal Revenue Code of 1986, as
amended, on defined contribution plans to which that section applies.

Accordingly, Union Oil Company of California, dba Unocal, hereby amends and
restates the Plan pursuant to the terms and provisions set forth below.

                                   ARTICLE 1

                                  DEFINITIONS

Wherever used herein, the following terms shall have the meanings hereafter set
forth:

1.1  "Accounting Date" means any date adopted by the Company for purposes of
     contributions to the Plan and interest accruals thereunder.

1.2  "Base Pay" shall have the same meaning ascribed to it in the Unocal Savings
     Plan except that the limitations under Section 401(a)(17) of the Code shall
     not apply to this Plan.

1.3  "Board" means the Board of Directors of the Company.

1.4  "Code" means the Internal Revenue Code of 1986, as amended from time to
     time, and any regulations relating thereto.

                                       1
<PAGE>

1.5  "Company" means Union Oil Company of California, dba Unocal, a California
     corporation, or, to the extent provided in Section 8.8 below, any successor
     corporation or other entity resulting from a merger or consolidation into
     or with the Company or a transfer or sale of substantially all of the
     assets of the Company.

1.6  "Member" means an employee of the Company or a subsidiary or affiliate
     thereof who is eligible for participation under the Qualified Plan and to
     whom or with respect to whom contributions may be made under the Plan.

1.7  "Plan" means the Unocal Supplemental Savings Plan.

1.8  "Plan Year" means the calendar year or any other 12-consecutive-month
     period that may be designated by the Company as its fiscal year and the
     fiscal year of the Qualified Plan.

1.9  "Qualified Plan" means the Unocal Savings Plan, and each predecessor,
     successor or replacement cash or deferred arrangement.

1.10 "Qualified Plan Company Matching Contribution" means the total of all
     matching contributions made by the Company for the benefit of a Member
     under and in accordance with the terms of the Qualified Plan in any Plan
     Year.

1.11 "Qualified Plan Accounts" means the accounts established for a Member
     under the Qualified Plan.

1.12 "Supplemental Matching Contribution" means the matching contribution
     credited to a Member's Supplemental Account by the Company for the benefit
     of a Member under and in accordance with the terms of the Plan in any Plan
     Year.

                                       2
<PAGE>

1.13  "Supplemental Account" means the account maintained by the Company under
      the Plan for a Member that is credited with amounts contributed under
      Section 3.1 of the Plan.

1.14  "Vested" shall have the same meaning as such term is defined under the
      Qualified Plan.

Words in the masculine gender shall include the feminine and the singular shall
include the plural, and vice versa, unless otherwise clearly indicated by the
context.  Headings used herein are included for ease of reference only, and are
not to be construed so as to alter the terms of the Plan.

                                  ARTICLE II

                                  ELIGIBILITY

A Member who is eligible to receive Qualified Plan Company Matching
Contributions, the total amount of which is reduced or would have been reduced
by reason of application of the limitations on contributions imposed under
Section 401(a)(17) of the Code, as in effect on any date for allocation of the
amount of the Qualified Plan Company Matching Contribution, or as in effect at
any time thereafter, to the Qualified Plan shall be eligible to participate in
the Plan.

                                  ARTICLE III

                      SUPPLEMENTAL MATCHING CONTRIBUTIONS

3.1  Supplemental Matching Contributions.

     Each Plan Year, the Company will make a Supplemental Matching Contribution
     to this Plan on behalf of each eligible Member in an amount equal to the
     excess, if any, of:

                                       3
<PAGE>

     (a)  Six percent (6%) of the Member's Base Pay for the Plan Year, but
          without giving effect to any reductions required by the limitations
          imposed by the Code on the Qualified Plan; over

     (b)  The amount of the Qualified Plan Company Matching Contributions that
          would have been allocated to the Member for the Plan Year if such
          Member elected to contribute six percent (6%) as a pre-tax
          contribution to said Plan.

3.2  At its sole discretion, the Company may elect to make any Supplemental
     Matching Contribution required above either as of the close of the Plan
     Year or as of an Accounting Date selected by the Company.

                                  ARTICLE IV

                    INTEREST ON SUPPLEMENTAL CONTRIBUTIONS

4.1  Amounts credited to a Member's Supplemental Account shall be credited with
     interest from the applicable Accounting Date until such credited amounts
     are distributed to the Member or his beneficiary.

4.2  For each Plan Year or portion thereof, interest shall be credited to each
     Member's Supplemental Account based on the average ten year U.S. Treasury
     bond rate for December of the year prior to such Plan Year, plus two
     percent (2%).

                                   ARTICLE V

                                 DISTRIBUTIONS

5.1  Distribution

     All amounts credited to a Participant's Supplemental Account which are
     Vested, including interest credited in accordance with Section 4.1 of the
     Plan, shall be distributed to or with respect to a Participant only upon
     termination of the

                                       4
<PAGE>

     Participant's employment with the Company and all affiliates thereof for
     any reason including death. All amounts distributable under the Plan shall
     be distributed in a single sum payment within 90 days following termination
     of employment with the Company and all affiliates.

     If a Member should die before distribution of the full amount of Member's
     Supplemental Account has been completed, any remaining amount shall be
     distributed to the beneficiary designated by the Member in writing
     delivered to the Company prior to his death.  If a Member has not
     designated a beneficiary or if no designated beneficiary is living on the
     date of distribution, such amounts shall be distributed in a single sum
     payment to those persons entitled to receive distributions of the Member's
     account under the Qualified Plan.

                                  ARTICLE VI

                          ADMINISTRATION OF THE PLANS

6.1  Administration by the Company

     The Company shall be responsible for the operation and administration of
     the Plan.

6.2  General Powers of Administration

     All provisions set forth in the Qualified Plan with respect to the
     administrative powers and duties of the Company, expenses of
     administration, and procedures for filing claims shall apply with respect
     to the Plan or the Qualified Plan.

     The Company shall be entitled to rely conclusively upon all tables,
     valuations, certificates, opinion and reports furnished by any actuary,
     accountant, controller, counsel or other person employed or engaged by the
     Company with respect to the Plan.

                                       5
<PAGE>

                                  ARTICLE VII

                           AMENDMENT OR TERMINATION

7.1  Amendment or Termination

     The Company intends the Plan to continue indefinitely but reserves the
     right to amend or terminate the Plan in whole or in part when, in the sole
     opinion of the Company, such amendment or termination is advisable.  Any
     such amendment or termination shall be made pursuant to a resolution of the
     Board and shall be effective as of the date of such resolution.

7.2  Effect of Amendment or Termination

     No amendment or termination of the Plan shall reduce the balance of any
     Supplemental Account held hereunder as of the effective date of such
     amendment or termination.  Upon termination of the Plan, distribution of
     amounts in Supplemental Account shall be made to each Member or a Member's
     beneficiary if the Member is deceased in a manner described in Section 5.1
     of the Plan.  In the event of such termination, all affected Employees
     shall become fully vested in the benefits payable hereunder and the Company
     may choose, in its discretion, to accelerate the payment of any such
     benefits.  No additional Supplemental Matching Contributions shall be made
     to the Supplemental Account of a Member after termination of the Plan, but
     the Company shall continue to credit interest pursuant to Section 4.1,
     until all amounts credited to the Member's Supplemental Account are
     distributed to the Member or the Member's beneficiary.

                                 ARTICLE VIII

                              GENERAL PROVISIONS

8.1  Member's Rights Unsecured.

     The right of a Member or his designated beneficiary to receive a
     distribution under the Plan shall be an unsecured claim against the general
     assets of the Company, and neither the Member nor a designated beneficiary
     shall have any

                                       6
<PAGE>

     rights in or against any specific assets of the Company. Nothing herein
     shall prohibit the Company from electing to provide benefits hereunder
     through any investment it elects, including life insurance policies or
     contracts or the establishment of a "Rabbi Trust." A Member shall have no
     interest or claims as to such investments, or trust or policies except as
     explicitly provided thereunder. Any investment returns, gains or losses of
     the Company shall not alter the interest credited to a Member under Section
     4.2 above.

8.2  General Conditions

     Except as otherwise expressly provided herein, all terms and conditions of
     the Qualified Plan applicable to a Qualified Plan Company Matching
     Contribution will also be applicable to a Supplemental Matching
     Contribution, or any other contributions to be made under the Qualified
     Plan, shall be made solely in accordance with the terms and conditions of
     the Qualified Plan and nothing in this Plan shall operate or be construed
     in any way to modify, amend or affect the terms and provisions of the
     Qualified Plan.

8.3  No Guarantee of Benefits

     Nothing contained in the Plan shall constitute a guaranty by the Company or
     any other person or entity that the assets of the Company will be
     sufficient to pay any benefit hereunder.

8.4  No Enlargement of Employee Rights

     Nothing in this Plan shall give any person a right to remain in the
     employment of the Employer or affect the right of the Employer to modify or
     terminate the employment of an Employee at any time, with or without cause.

8.5  Spendthrift Provision

     No interest of any person or entity in, or right to receive a distribution
     under, the Plan shall be subject in any manner to sale, transfer,
     assignment, pledge, attachment, garnishment, or other alienation or
     encumbrance of any kind; nor may such interest or right to receive a
     distribution be taken, either voluntarily or

                                       7
<PAGE>

     involuntarily for the satisfaction of the debts of, or other obligations or
     claims against, such person or entity, including claims for alimony,
     support, separate maintenance and claims in bankruptcy proceedings.

8.6  Applicable Law

     The Plan shall be construed and administered under the laws of the State of
     California.

8.7  Incapacity of Recipient

     If any person entitled to a distribution under the Plan is deemed by the
     Company to be incapable of personally receiving and giving a valid receipt
     for such payment, then, unless and until claim therefor shall have been
     made by a duly appointed guardian or other legal representative of such
     person, the Company may provide for such payment or any part thereof to be
     made to any other person or institution then contributing toward or
     providing for the care and maintenance of such person.  Any such payment
     shall be a payment for the account of such person and a complete discharge
     of any liability of the Company and the Plan therefor.

8.8  Corporate Successors

     The Plan shall not be automatically terminated by a transfer or sale of
     assets of the Company or by the merger or consolidation of the Company into
     or with any other corporation or other entity, but the Plan shall be
     continued after such sale, merger or consolidation only if and to the
     extent that the transferee, purchaser or successor entity agrees to
     continue the Plan.  In the event that the Plan is not continued by the
     transferee, purchaser or successor entity, then the Plan shall terminate
     subject to the provisions of Section 7.2.

8.9  Unclaimed Benefit

     Each Member shall keep the Company informed of his current address and the
     current address of his designated beneficiary.  The Company shall not be
     obligated to search for the whereabouts of any person.  If the location of
     a

                                       8
<PAGE>

     Member is not made known to the Company within three (3) years after the
     date on which distribution of the Member's Supplemental Account may first
     be made, distribution may be made as though the Member had died at the end
     of the three-year period. If, within one additional year after such three-
     year period has elapsed, or, within three years after the actual death of a
     Member, the Company is unable to locate any designated beneficiary of the
     Member, then the Company shall have no further obligation to pay any
     benefit hereunder to such Member or designated beneficiary and such benefit
     shall be irrevocably forfeited.

8.10 Dispute Resolution

     Any controversy or claim arising out of or relating to this Plan shall be
     settled by binding arbitration in Los Angeles, California, in accordance
     with the Commercial Arbitration Rules of the American Arbitration
     Association.  The parties shall seek to agree upon appointment of the
     arbitrator and the arbitration procedures.  If the parties are unable to
     reach such agreement, a single arbitrator who is a retired judge of a
     Federal or California state court shall be appointed pursuant to the AAA
     Commercial Arbitration Rules, and the arbitrator shall determine the
     arbitration procedures.  Any award pursuant to such arbitration shall be
     included in a written decision which shall state the legal and factual
     reasons upon which the award was based, including all the elements involved
     in the calculation of any award.  Any such award shall be deemed final and
     binding and may be entered and enforced in any state or federal court of
     competent jurisdiction.  The arbitrator shall interpret the Plan in
     accordance with the laws of California.  The arbitrator shall be authorized
     to award reasonable attorney's fees and other arbitration-related costs to
     a Member or beneficiary.  The award shall be limited to Plan benefits at
     issue, reasonable attorney's fees and arbitration-related costs.

8.11 Limitations on Liability

     Notwithstanding any of the preceding provisions of the Plan, neither the
     Company nor any individual acting as employee or agent of the Company shall
     be liable to any Member, former Member or other person for any claim, loss,
     liability or expense incurred in connection with the Plan.

                                       9