<PAGE>
 
                                                                   EXHIBIT 10(n)


                              SECOND AMENDMENT TO
                  FOURTH AMENDED AND RESTATED CREDIT AGREEMENT

          THIS SECOND AMENDMENT TO FOURTH AMENDED AND RESTATED CREDIT AGREEMENT
(this "Amendment") dated as of the 15th day of March, 1999, by and among PLAINS
RESOURCES INC., a Delaware corporation (the "Company"), ING (U.S.) CAPITAL LLC,
successor in interest to ING (U.S.) CAPITAL CORPORATION, as Agent ("Agent"), and
the Lenders under the Original Agreement (as defined herein).

                              W I T N E S S E T H:

          WHEREAS,  the Company, Agent and Lenders entered into that certain
Fourth Amended and Restated Credit Agreement dated as of May 22, 1998, as
amended by a First Amendment to Fourth Amended and Restated Credit Agreement
dated November 17, 1998 (as amended, the "Original Agreement") for the purposes
and consideration therein expressed, pursuant to which Lenders became obligated
to make and made loans to the Company as therein provided; and

          WHEREAS, the Company, Agent and Lenders desire to amend the Original
Agreement for the purposes described herein;

          NOW, THEREFORE, in consideration of the premises and the mutual
covenants and agreements contained herein and in the Original Agreement, in
consideration of the loans which may hereafter be made by Lenders to the
Company, and for other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties hereto do hereby agree
as follows:

                    ARTICLE I. -- Definitions and References

          (S) 1.1.  Terms Defined in the Original Agreement.  Unless the context
otherwise requires or unless otherwise expressly defined herein, the terms
defined in the Original Agreement shall have the same meanings whenever used in
this Amendment.

          (S) 1.2.  Other Defined Terms.  Unless the context otherwise requires,
the following terms when used in this Amendment shall have the meanings assigned
to them in this (S) 1.2.

          "Amendment" means this Second Amendment to Fourth Amended and Restated
     Credit Agreement.

          "Amendment Documents" means this Amendment.

          "Credit Agreement" means the Original Agreement as amended hereby.

                                      -1-
<PAGE>
 
                           ARTICLE II. -- Amendments

     (S) 2.1.   Financial Statements.  The references to "Consolidated
Subsidiaries" in clause (i) of Section 8.01(a) and clause (i) of Section 8.01(b)
are hereby amended to refer instead to "Consolidated Subsidiaries and
Unrestricted Subsidiaries".

     (S) 2.2.   Investments.  Section 8.10(f) of the Original Agreement is
hereby amended in its entirety to read as follows:

          (f) in addition to any capital contributions permitted in subsection
     (e) above, the following Investments in Unrestricted Subsidiaries:   (i)
     capital contributions of up to $85,000,000 of the proceeds of any preferred
     or common stock of the Company issued after January 1, 1998 and prior to
     December 31, 1998, (ii) any Investment represented by, or required to
     comply with the obligations undertaken under, the Stock Purchase Agreement
     dated as of March 15, 1998 among the Company, PAAI and Wingfoot Ventures
     Seven Inc., as amended or modified, made prior to December 31, 1998, and
     (iii) on or prior to December 31, 1999, an aggregate Investment of up to
     $40,000,000 in one or more Unrestricted Subsidiaries in connection with
     their acquisition of, and respective Investments in, Scurlock Permian LLC,
     a wholly-owned subsidiary of Marathon Ashland Petroleum LLC, which is
     engaged in crude oil transportation, trading and marketing in an area
     reaching from the Rocky Mountains through the Gulf Coast.

     (S) 2.3.   Current Ratio.  Section 8.13 of the Original Agreement is hereby
amended in its entirety to read as follows:

          8.13  Current Ratio.  The Company will not at any time permit current
     assets of the Company and its Consolidated Subsidiaries to be less than
     100% of current liabilities.  For purposes hereof, the terms "current
     assets" and "current liabilities" shall have the respective meanings
     assigned to them by GAAP, and, in addition (i) the unused amount of the
     Borrowing Base plus sixty percent (60%) of the fair market value of any
     common units of the MLP owned directly or indirectly by the Company shall
     be included as current assets, and (ii) all LC Obligations shall be
     included as current liabilities, regardless of whether or not contingent
     (but without duplication).

     (S) 2.4.   Use of Proceeds.  The last sentence of Section 8.17 of the
Original Agreement is hereby amended to read as follows:

     In addition, the Company may use up to $55,000,000 of the proceeds of the
     Loans hereunder to make or refinance capital contributions to Unrestricted
     Subsidiaries as permitted in Section 8.10(e) and up to $40,000,000 of the
     proceeds of the Loans hereunder to make Investments to one or more
     Unrestricted Subsidiaries as permitted in Section 8.10(f)(iii).

                  ARTICLE III. -- Conditions of Effectiveness

     (S) 3.1.  Effective Date.  This Amendment shall become effective as of the
date first above written when and only when Agent shall have received, at
Agent's office, a counterpart of this Amendment executed and delivered by the
Company, Agent and each Lender.

                                      -2-
<PAGE>
 
                 ARTICLE IV. -- Representations and Warranties

     (S) 4.1.  Representations and Warranties of the Company.  In order to
induce Agent and Lenders to enter into this Amendment, the Company represents
and warrants to Agent and Lenders that:

          (a) The representations and warranties contained in Section 7 of the
     Original Agreement, are true and correct at and as of the time of the
     effectiveness hereof, subject to the amendment of certain of the Schedules
     to the Credit Agreement as attached hereto.

          (b) The Company and the Subsidiaries are duly authorized to execute
     and deliver this Amendment and the other Amendment Documents to the extent
     a party thereto, and the Company is and will continue to be duly authorized
     to borrow and perform its obligations under the Credit Agreement.  The
     Company and the Subsidiaries have duly taken all corporate action necessary
     to authorize the execution and delivery of this Amendment and the other
     Amendment Documents, to the extent a party thereto, and to authorize the
     performance of their respective obligations thereunder.

          (c) The execution and delivery by the Company and the Subsidiaries of
     this Amendment and the other Amendment Documents, to the extent a party
     thereto, the performance by the Company and the Subsidiaries of their
     respective obligations hereunder and thereunder, and the consummation of
     the transactions contemplated hereby and thereby, do not and will not
     conflict with any provision of law, statute, rule or regulation or of the
     certificate or articles of incorporation and bylaws of the Company or any
     Subsidiary, or of any material agreement, judgment, license, order or
     permit applicable to or binding upon the Company or any Subsidiary, or
     result in the creation of any lien, charge or encumbrance upon any assets
     or properties of the Company or any Subsidiary, except in favor of Agent
     for the benefit of Lenders.  Except for those which have been duly
     obtained, no consent, approval, authorization or order of any court
     or governmental authority or third party is required in connection with the
     execution and delivery by the Company or any Subsidiary of this Amendment
     or any other Amendment Document, to the extent a party thereto, or to
     consummate the transactions contemplated hereby and thereby.

          (d) When this Amendment and the other Amendment Documents have been
     duly executed and delivered, each of the Basic Documents, as amended by
     this Amendment and the other Amendment Documents, will be a legal and
     binding instrument and agreement of the Company and the Subsidiaries, to
     the extent a party thereto, enforceable in accordance with its terms,
     (subject, as to enforcement of remedies, to applicable bankruptcy,
     insolvency and similar laws applicable to creditors' rights generally and
     to general principles of equity).

                          ARTICLE V. -- Miscellaneous

     (S) 5.1.  Ratification of Agreements.  The Original Agreement, as hereby
amended, is hereby ratified and confirmed in all respects.  The Basic Documents,
as they may be amended or 

                                      -3-
<PAGE>
 
affected by this Amendment and/or the other Amendment Documents, are hereby
ratified and confirmed in all respects. Any reference to the Credit Agreement in
any Basic Document shall be deemed to refer to this Amendment also. The
execution, delivery and effectiveness of this Amendment and the other Amendment
Documents shall not, except as expressly provided herein or therein, operate as
a waiver of any right, power or remedy of Agent or any Lender under the Credit
Agreement or any other Basic Document nor constitute a waiver of any provision
of the Credit Agreement or any other Basic Document.

     (S) 5.2.  Ratification of Security Documents.  The Company, Agent and
Lenders each acknowledge and agree that any and all indebtedness, liabilities or
obligations arising under or in connection with the Notes are Obligations and is
secured indebtedness under, and is secured by, each and every Security Document
to which the Company is a party.  The Company hereby re-pledges, re-grants and
re-assigns a security interest in and lien on every asset of the Company
described as collateral in any Security Document.

     (S) 5.3.  Survival of Agreements.  All representations, warranties,
covenants and agreements of the Company herein and in the other Amendment
Documents shall survive the execution and delivery of this Amendment and the
other Amendment Documents and the performance hereof and thereof, including
without limitation the making or granting of each Loan, and shall further
survive until all of the Obligations are paid in full.  All statements and
agreements contained in any certificate or instrument delivered by the Company
or any Subsidiary hereunder, under the other Amendment Documents or under the
Credit Agreement to Agent or any Lender shall be deemed to constitute
representations and warranties by, or agreements and covenants of, the Company
under this Amendment and under the Credit Agreement.

     (S) 5.4.  Basic Documents.  This Amendment and each of the other Amendment
Documents is a Basic Document, and all provisions in the Credit Agreement
pertaining to Basic Documents apply hereto and thereto.

     (S) 5.5.  GOVERNING LAW.  THIS AMENDMENT AND THE OTHER AMENDMENT DOCUMENTS
SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF
NEW YORK AND ANY APPLICABLE LAWS OF THE UNITED STATES OF AMERICA IN ALL
RESPECTS, INCLUDING CONSTRUCTION, VALIDITY AND PERFORMANCE.

     (S) 5.6.  Counterparts.  This Amendment and each of the other Amendment
Documents may be separately executed in counterparts and by the different
parties hereto in separate counterparts, each of which when so executed shall be
deemed to constitute one and the same Amendment or Amendment Document, as the
case may be.

                                      -4-
<PAGE>
 
     IN WITNESS WHEREOF, this Amendment is executed as of the date first above
written.

                                         PLAINS RESOURCES INC.

                                         By:   /s/ Michael R.  Patterson
                                            --------------------------------    
                                            Michael R.  Patterson
                                            Vice President and General Counsel

                                      -5-
<PAGE>
 
                                         ING (U.S.) CAPITAL LLC,
                                         as Agent and a Lender

                                         By:   /s/ Peter Y.  Clinton
                                            --------------------------------    
                                            Name: Peter Y.  Clinton
                                            Title:   Senior Vice President

                                      -6-
<PAGE>
 
                                         BANKBOSTON, N.A., Lender

                                         By:   /s/ Terrence Ronan
                                            --------------------------------    
                                            Terrence Ronan, Vice President


 

                                      -7-
<PAGE>
 
                                         DEN NORSKE BANK ASA, Lender

                                         By:   /s/ J.  Morten Kreutz
                                            --------------------------------    
                                            Name: J.  Morten Kreutz
                                            Title:   Vice President

                                         By:   /s/ William V.  Moyer
                                            --------------------------------    
                                            Name: William V.  Moyer
                                            Title:   Senior Vice President


 

                                      -8-
<PAGE>
 
                                         WELLS FARGO BANK (TEXAS),
                                         NATIONAL ASSOCIATION, Lender

                                         By:   /s/ Ann M.  Rhoads
                                            --------------------------------    
                                            Name: Ann M.  Rhoads
                                            Title:   Vice President

                                      -9-
<PAGE>
 
                                         CHASE BANK OF TEXAS, N.A., Lender

                                         By:   /s/ Russell A.  Johnson
                                            --------------------------------    
                                            Name: Russell A.  Johnson
                                            Title:   Vice President


 

                                      -10-
<PAGE>
 
                                         COMERICA BANK-TEXAS, Lender

                                         By:   /s/ Daniel G.  Steele
                                            --------------------------------    
                                            Name: Daniel G.  Steele
                                            Title:   Senior Vice President


 

                                      -11-
<PAGE>
 
                                         MEESPIERSON CAPITAL CORP., Lender

                                         By:      /s/ Darrell W.  Holley
                                            --------------------------------    
                                            Name: Darrell W.  Holley
                                            Title:   Senior Vice President

                                         By:    /s/ Karel Louman
                                            --------------------------------    
                                            Name: Karel Louman
                                            Title:   Managing Director


 

                                      -12-
<PAGE>
 
                                         BANK OF SCOTLAND, Lender

                                         By:   /s/ Annie Chin Tat
                                            --------------------------------    
                                            Name: Annie Chin Tat
                                            Title:   Senior Vice President


 

                                      -13-
<PAGE>
 
                                         U.S. BANK NATIONAL ASSOCIATION, Lender

                                         By:   /s/ Monte E.  Deckerd
                                            --------------------------------    
                                            Name: Monte D.  Deckerd
                                            Title:   Vice President


 

                                      -14-
<PAGE>
 
                                         HIBERNIA NATIONAL BANK

                                         By:   /s/ Tammy Angelety
                                            --------------------------------    
                                            Name: Tammy Angelety
                                            Title:   Vice President

                                      -15-
<PAGE>
 
                                        GENERAL ELECTRIC CAPITAL CORPORATION

                                         By:  /s/ Michael J. Tzougrakis
                                            --------------------------------    
                                            Name:  Michael J. Tzougrakis
                                            Title: Manager of Operations

                                      -16-
<PAGE>
 
                             CONSENT AND AGREEMENT

          Each of the undersigned Subsidiary Guarantors hereby consents to the
provisions of this Amendment and the transactions contemplated herein and hereby
(i) acknowledges and agrees that any and all indebtedness, liabilities or
obligations arising under or in connection with the Notes are Obligations and
are secured indebtedness under, and are secured by, each and every Security
Document to which it is a party, (ii) re-pledges, re-grants and re-assigns a
security interest in and lien on all of its assets described as collateral in
any Security Document, (iii) ratifies and confirms its Amended and Restated
Guaranty dated May 22, 1998 made by it for the benefit of Agent and Lenders, and
(iv) expressly acknowledges and agrees that such Subsidiary Guarantor guarantees
all indebtedness, liabilities and obligations arising under or in connection
with the Notes pursuant to the terms of such Amended and Restated Guaranty, and
agrees that its obligations and covenants thereunder are unimpaired hereby and
shall remain in full force and effect.

                                    PLAINS RESOURCES INTERNATIONAL INC.
                                    STOCKER RESOURCES, INC.
                                    CALUMET FLORIDA, INC.
                                    PLAINS ILLINOIS INC.


                                    By:   /s/ Michael R. Patterson
                                       --------------------------------    
                                       Name:  Michael R. Patterson
                                       Title: Vice President

                                    STOCKER RESOURCES, L.P.

                                    By:  Stocker Resources, Inc.,
                                         its General Partner


                                         By:   /s/ Michael R. Patterson
                                            --------------------------------    
                                            Name:  Michael R. Patterson
                                            Title: Vice President

                                      -17-