<PAGE>

                                                                 Exhibit (10)(z)

                            THE LUBRIZOL CORPORATION
                        2005 EXCESS DEFINED BENEFIT PLAN

      The Lubrizol Corporation hereby establishes, effective January 1, 2005,The
Lubrizol Corporation 2005 Excess Defined Benefit Plan (the "Plan") for the
purpose of providing supplemental benefits to certain employees, as permitted by
Section 3(36) of the Employee Retirement Income Security Act of l974.

                                    ARTICLE I

                          DEFINITIONS AND CONSTRUCTION

      1.1 Definitions. For the purposes hereof, the following words and phrases
shall have the meanings indicated, unless a different meaning is plainly
required by the context:

            (a) Code. the term "Code" shall mean the Internal Revenue Code as
      amended from time to time. Reference to a section of the Code shall
      include such section and any comparable section or sections of any future
      legislation that amends, supplements, or supersedes such section.

            (b) Company. The term "Company" shall mean The Lubrizol Corporation,
      an Ohio corporation, its corporate successors and the surviving
      corporation resulting from any merger of The Lubrizol Corporation with any
      other corporation or corporations, and any subsidiaries of The Lubrizol
      Corporation which adopt the Plan.

            (c) Lubrizol Pension Plan. The term "Lubrizol Pension Plan" shall
      mean The Lubrizol Corporation Pension Plan as the same shall be in effect
      on the date of a Participant's retirement, death, or other termination of
      employment.

            (d) Participant. the term "Participant" shall mean any person
      employed by the Company who is listed on Appendix A attached hereto, or
      who is designated by the Board of Directors as an officer for the purposes
      of Section 16 of the Securities Exchange Act of 1934, or whose benefits
      under the Lubrizol Pension Plan are limited by the application of Section
      401(a)(17) of the Internal Revenue Code of 1986, as amended.

            (e) Plan. The term "Plan" shall mean the excess defined benefit
      pension plan as set forth herein, together with all amendments hereto,
      which Plan shall be called "The Lubrizol Corporation 2005 Excess Defined
      Benefit Plan."

            (f) Trust. The term "Trust" shall mean The Lubrizol Corporation
      Excess Defined Benefit Plan Trust established pursuant to the Trust
      Agreement.

            (g) Trust Agreement. The term "Trust Agreement" shall mean The
      Lubrizol Corporation Excess Defined Benefit Plan Trust Agreement.

      1.2. Additional Definitions. All other words and phrases used herein shall
have the meanings given them in the Lubrizol Pension Plan, unless a different
meaning is clearly required by the context.

<PAGE>

                                   ARTICLE II

                          SUPPLEMENTAL PENSION BENEFIT

      2.1 Eligibility. A Participant who separates from service with the Company
and its subsidiaries and

            (a) whose benefits under the Lubrizol Pension Plan are limited by
      the provisions of Section 401(a)(17) or 415 of the Code,

            (b) who participated in The Lubrizol Corporation 2005 Deferred
      Compensation Plan for Officers, or

            (c) who participated in The Lubrizol Corporation 2005 Executive
      Council Deferred Compensation Plan

shall be eligible for a supplemental pension benefit determined in accordance
with the provisions of Section 2.2.

      2.2 Amount. Subject to the provisions of Article III, the monthly
supplemental pension benefit payable to an eligible Participant shall be an
amount which when added to the monthly pension payable to such Participant under
the Lubrizol Pension Plan (prior to any reduction applicable to an optional
method of payment) equals the monthly pension benefit which would have been
payable under the Lubrizol Pension Plan (prior to any reduction applicable to an
optional method of payment and adjusted for any amount payable under The
Lubrizol Corporation 2005 Excess Defined Contribution Plan which is attributable
to The Lubrizol Corporation Employees' Profit Sharing Plan and Savings Plan and
which would have affected the benefit that the Participant would have received
under the Lubrizol Pension Plan had it been payable from The Lubrizol
Corporation Employees' Profit Sharing and Savings Plan) if the limitations of
Section 401(a)(17) and 415 of the Code were not in effect and, (if he is a
Participant described in Section 2.1(b) or (c)), if he did not participate in
The Lubrizol Corporation 2005 Deferred Compensation Plan for Officers or in The
Lubrizol Corporation 2005 Executive Council Deferred Compensation Plan.

      2.3 Vesting. Each Participant shall be vested in his supplemental pension
benefit under this Plan as determined in accordance with the vesting provisions
of the Lubrizol Pension Plan.

                                   ARTICLE III

                               PAYMENT OF BENEFITS

      3.1 Payment to Participant.

3.2 Distribution.

            (a) Each Participant who separates from service with the Company and
      its related corporations shall receive payment of his supplemental pension
      benefit in the standard form of payment of a single lump-sum payment
      payable the later of six months following the separation from service or
      30 days following the calendar year in which Participant separated from
      service.

<PAGE>

            (b) At least 12 months prior to the distribution date specified in
      paragraph (a) Participants may instead elect to receive the actuarial
      equivalent of the benefit determined under Section 2.2 on the date of
      separation from service, and payable commencing five years after the
      distribution date specified in paragraph (a) above in accordance with any
      one of the following options:

                  (i) Payments will be made to the Participant for his lifetime
            with the continuance to his Beneficiary of such amount after his
            death for the remainder, if any, of the 120-month term that
            commenced with the date as of which the first payment of such
            monthly benefit is made, and with any such monthly benefits
            remaining unpaid upon the death of the survivor of the Participant
            and his Beneficiary to be made to the estate of such survivor.

                  (ii) A reduced monthly retirement benefit payable to such
            Participant for his lifetime with the continuance of a monthly
            benefit equal to fifty percent (50%) of such reduced amount after
            his death to the Participant's Beneficiary during the lifetime of
            the Beneficiary, provided that such Beneficiary is living at the
            time of such Participant's separation from service and survives such
            Participant.

                  (iii) A reduced monthly retirement benefit payable to such
            Participant during his lifetime with the continuance of a monthly
            benefit equal to one hundred percent (100%) of such reduced amount
            after his death to the Participant's Beneficiary during the lifetime
            of the Beneficiary, provided such Beneficiary is living at the time
            of such Participant's separation from service and survives such
            Participant.

      The forms of payment described shall be calculated using the same
actuarial factors and interest rates used under The Lubrizol Corporation Pension
Plan (or its successor) as in effect on the date of separation from service.

      3.2 Payment in the Event of Death Prior to Commencement of Distribution.
If a Participant dies prior to commencement of benefits under the Plan, his
surviving spouse, if any, shall be eligible for a survivor benefit which is
equal to one-half of the reduced monthly benefit the Participant would have
received under the Plan if the Participant had retired on the day before his
death and had elected to receive his benefit under the Lubrizol Pension Plan in
a 50 percent joint and survivor annuity form. In making the determinations and
reductions required in this Section 3.2, the Company shall apply the assumptions
then in use under the Lubrizol Pension Plan. For purposes hereof, a surviving
spouse shall only be eligible for a benefit under this Section 3.2, if such
spouse had been married to the deceased Participant for at least one year as of
the date of the Participant's death.

                                   ARTICLE IV

                                 ADMINISTRATION

      4.1 Authority of the Company. The Company shall be responsible for the
general administration of the Plan, for carrying out the provisions hereof, and
for making, or causing the

<PAGE>

Trust to make, any required supplemental benefit payments. The Company shall
have all such powers as may be necessary to carry out the provisions of the
Plan, including the power to determine all questions relating to eligibility for
and the amount of any supplemental pension benefit and all questions pertaining
to claims for benefits and procedures for claim review; to resolve all other
questions arising under the Plan, including any questions of construction; and
to take such further action as the Company shall deem advisable in the
administration of the Plan. The Company may delegate any of its powers,
authorities, or responsibilities for the operation and administration of the
Plan to any person or committee so designated in writing by it and may employ
such attorneys, agents, and accountants as it may deem necessary or advisable to
assist it in carrying out its duties hereunder. The actions taken and the
decisions made by the Company hereunder shall be final and binding upon all
interested parties.

      4.2 Claims Review Procedure. The Company shall notify the person who files
a claim for benefits (hereinafter referred to as the "Claimant") of the Plan's
adverse benefit determination within a reasonable period of time, but not later
than 90 days after the receipt of the claim by the Plan, unless the Company
determines that special circumstances require an extension of time for
processing the claim. If the Company determines that special circumstances
require an extension of time for processing is required, written notice of the
extension shall be furnished to the Claimant prior to the termination of the
initial 90-day period. In no event shall such extension exceed a period of 90
days from the end of such initial period. The extension notice shall indicate
the special circumstances requiring an extension of time and the date by which
the Plan expects to render the benefit determination. Whenever the Company
decides for whatever reason to deny, whether in whole or in part, a claim for
benefits filed by any Claimant, the Company shall transmit to the Claimant a
written notice of the Company's decision, which shall be written in a manner
calculated to be understood by the Claimant and contain a statement of the
specific reasons for the denial of the claim, reference to the specific Plan
provisions on which the determination was based, a description of any additional
material or information necessary for the Claimant to perfect the claim and an
explanation of why such material or information is necessary, a description of
the Plan's review procedures and the time limits applicable to such procedures,
include a statement of the Claimant's right to bring civil action under Section
502(a) ERISA following an adverse benefit determination on review. Within 60
days of the date on which the Claimant receives such notice, he or his
authorized representative may request that the claim denial be reviewed by
filing with the Company a written request therefor, which request shall contain
the following information:

            (a) the date on which the Claimant's request was filed with the
            Company; provided, however, that the date on which the Claimant's
            request for review was in fact filed with the Company shall control
            in the event that the date of the actual filing is later than the
            date stated by the Claimant pursuant to this paragraph (a);

            (b) the specific portions of the denial of his claim which the
            Claimant requests the Company to review;

            (c) a statement by the Claimant setting forth the basis upon which
            he believes the Company should reverse the Company's previous denial
            of his claim for benefits and accept his claim as made; and

<PAGE>

            (d) any written comments, documents, records and other information
            which the Claimant desires the Company to examine in its
            consideration of his position as stated pursuant to paragraph (c).

Claimant shall be provided, upon request and free of charge, reasonable access
to, and copies of, all documents, records, and other information relevant to the
Claimant's claim for benefits. The review of the claim will take into account
all comments, documents, records and other information submitted by the Claimant
relating to the claim, without regard to whether such information was submitted
or considered in the initial benefit determination. Within no later than 60 days
of the date determined pursuant to paragraph (a) of this Section 4.2, the
Company shall notify Claimant of the Plan's benefit determination, unless the
Company determines that special circumstances require an extension of time for
processing the claim. If the Company determines that an extension of time for
processing is required, written notice of the extension will be furnished to the
Claimant prior to the termination of the initial 60-day period. In no event
shall such extension exceed a period of 60 days from the end of the initial
period. The extension notice shall indicate the special circumstances requiring
an extension of time and the date by which the Plan expects to render the
determination on review. The Company shall provide the Claimant with a written
notification of the Plan's benefit determination on review, written in a manner
calculated to be understood by the Claimant, including the reasons and Plan
provisions upon which its decision was based, a statement that the Claimant is
entitled to receive, upon request and free of charge, reasonable access to, and
copies of, all documents, records and other information relevant to the
Claimant's claim for benefits, and a statement of the Claimant's right to bring
an action under Section 502(a) of ERISA.

                                    ARTICLE V

                            AMENDMENT AND TERMINATION

      The Company reserves the right to amend or terminate the Plan in whole or
in part at any time and to suspend operation of the Plan, in whole or in part,
at any time, by resolution or written action of its Board of Directors or by
action of a committee to which such authority has been delegated by the Board of
Directors; provided, however, that no amendment shall result in the forfeiture
or reduction of the interest of any Participant or person claiming under or
through any one or more of them pursuant to the Plan. Any amendment of the Plan
shall be in writing and signed by authorized individuals.

                                   ARTICLE VI

                                  MISCELLANEOUS

      6.1 Non-Alienation of Retirement Rights or Benefits. No Participant shall
encumber or dispose of his right to receive any payments hereunder, which
payments or the right thereto are expressly declared to be non-assignable and
non-transferable. If a Participant attempts to assign, transfer, alienate or
encumber his right to receive any payment hereunder or permits the same to be
subject to alienation, garnishment, attachment. execution, or levy of any kind,
then thereafter during the life of such Participant, and also during any period
in which any Participant is incapable in the judgment of the Company of
attending to his financial affairs, any payments which the Company is required
to make hereunder may be made, in the discretion of the Company, directly to
such Participant or to any other person for his use or benefit or that of his
dependents, if any, including any

<PAGE>

person furnishing goods or services to or for his use or benefit or the use or
benefit of his dependents, if any. Each such payment may be made without the
intervention of a guardian, the receipt of the payee shall constitute a complete
acquittance to the Company with respect thereto, and the Company shall have no
responsibility for the proper allocation thereof.

      6.2 Plan Non-Contractual. Nothing herein contained shall be construed as a
commitment or agreement on the part of any person employed by the Company to
continue his employment with the Company, and nothing herein contained shall be
construed as a commitment on the part of the Company to continue the employment
or the annual rate of compensation of any such person for any period, and all
Participants shall remain subject to discharge to the same extent as if the Plan
had never been established.

      6.3 Trust. In order to provide a source of payment for its obligations
under the Plan, the Company has established the Trust, the terms of which are
governed by the Trust Agreement.

      6.4 Interest of a Participant. Subject to the provisions of the Trust
Agreement, the obligation of the Company under the Plan to provide a Participant
with a supplemental pension benefit constitutes the unsecured promise of the
Company to make payments as provided herein, and no person shall have any
interest in, or a lien or prior claim upon, any property of the Company.

      6.5 Controlling Status. No Participant shall be eligible for a benefit
under the Plan unless such Participant is a Participant on the date of his
retirement, death, or other termination of employment.

      6.6 Claims of Other Persons. The provisions of the Plan shall in no event
be construed as giving any person, firm or corporation any legal or equitable
right as against the Company, its officers, employees, or directors, except any
such rights as are specifically provided for in the plan or are hereafter
created in accordance with the terms and provisions of the Plan.

      6.7 Severability. The invalidity or unenforceability of any particular
provision of the Plan shall not affect any other provision hereof, and the Plan
shall be construed in all respects as if such invalid or unenforceable provision
were omitted herefrom.

      6.8 Governing Law. The provisions of the Plan shall be governed and
construed in accordance with the laws of the State of Ohio.

<PAGE>

                                   APPENDIX A
                                       TO
                            THE LUBRIZOL CORPORATION
                           EXCESS DEFINED BENEFIT PLAN

<TABLE>
<CAPTION>
Participants                                          Effective Date
------------                                          --------------
<S>                                                   <C>
</TABLE>