<PAGE>

                                                                   EXHIBIT 10(t)

 
                          Dated as of August 29, 1997


Plains Marketing & Transportation Inc.
1600 Smith Street
Houston, TX 77002

  Re:  Amendment No. 6 to Uncommitted Secured
       Demand Transactional Line of Credit Facility
       --------------------------------------------

Gentlemen:

  Reference is made to that certain letter agreement outlining the parameters of
an uncommitted secured demand transactional line of credit facility dated August
23, 1995 (as further amended to date and including all exhibits, schedules and
annexes thereto, the "Marketing Letter Agreement") among BankBoston, N.A. (f/k/a
The First National Bank of Boston) ("BKB"), Internationale Nederlanden (U.S.)
Capital Corporation ("ING"), Den Norske Bank ASA, Comerica Bank-Texas, Wells
Fargo Bank (Texas), National Association and such other banks as may from time
to time become parties thereto, (collectively, the "Lenders") and BKB, as agent
for the Lenders (in such capacity, the "Agent") and Plains Marketing &
Transportation Inc. (the "Borrower").  All capitalized terms used herein without
definition which are defined in the Marketing Letter Agreement shall have the
same meaning herein as therein.

  The Borrower, the Lenders and the Agent wish to amend certain terms of the
Marketing Letter Agreement as follows:

  1.     FINANCIAL AND OTHER GUIDELINES.  Schedule 4 to the Marketing Letter
Agreement is hereby amended as follows:

  (a)    paragraph (xi) of Schedule 4 is amended and restated in its entirety to
read as follows:

  "(xi)  Borrower shall not create, assume, suffer to exist or incur any
Indebtedness other than Indebtedness incurred in connection with the
Accommodations and Indebtedness as a guarantor of obligations of Resources
arising under the Resource Credit Agreement and that certain Indenture dated as
of March 15, 1996 among Resources, certain subsidiaries of Resources and Texas
Commerce Bank National Association as Trustee, pursuant to which Resources
issued 10 1/4% Senior Subordinated Notes due 2006, Series A and Series B, in the
aggregate principle amount of $150,000,000 and that certain Indenture dated as
of July 21, 1997 among Resources, certain subsidiaries of Resources and Texas
Commerce Bank National Association as Trustee,
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pursuant to which Resources issued 10 1/4% Senior Subordinated Notes due 2006,
Series C and Series D, in the aggregate principal amount of $50,000,000 and an
unsecured inter-company line of credit provided by Resources to the Borrower in
the amount of $15,000,000, the proceeds of which are to be used to finance the
purchase and physical storage of crude oil which is fully hedged on the NYMEX
and located in the Cushing Terminal or which is in transit in specified
pipelines approved by the Lenders and listed on Schedule 5 hereto.
"INDEBTEDNESS" means all obligations contingent or otherwise incurred in
connection with the borrowing of money or the extension of credit (other than
trade debt incurred on an open account basis customarily extended in the
ordinary course of business in connection with normal purchases of goods and
services) including without limitation capitalized lease obligations (except
such obligations respecting leases of office computers and similar general
purpose supplies and equipment leases in an amount not to exceed $50,000 and
obligations respecting truck leases) and guaranties or other contingent
obligations relating to obligations of others which would be classified as
Indebtedness".

     2.  SECURITY.  The Borrower hereby confirms that the reference to
"Marketing Letter Agreement" in the term "Marketing Obligations" as used in that
certain Security Agreement dated as of August 23, 1995 between the Borrower and
the Agent includes the Marketing Letter Agreement as amended hereby and that
references to the Demand Loans and L/C's issued pursuant to the Marketing Letter
Agreement refers to all Demand Loans and L/C's issued pursuant to the Marketing
Letter Agreement, as amended hereby.

     3.  CONDITIONS PRECEDENT.  This Amendment shall become effective upon
receipt by the Agent of a counterpart of this Amendment duly signed where
indicated below by each Lender, the Agent, the Borrower and the Plains Resources
Inc.
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                                      -3-

     If you agree to and accept the foregoing amendment, please so indicate by
signing a counterpart of this letter and returning it to the Agent.  Upon
satisfaction of the conditions set forth in Section 3 hereof, this Amendment
shall take effect as a binding agreement among us, to be construed and
enforceable in accordance with the laws of The Commonwealth of Massachusetts.

                                 PLAINS MARKETING &                
                                 TRANSPORTATION INC.               
                                                                   
                                                                   
                                 By:  /s/MICHAEL R. PATTERSON      
                                 Name:   Michael R. Patterson      
                                 Title:  Vice President            
                                                                   
                                                                   
                                 BANKBOSTON, N.A.,                 
                                 Individually and as Agent         
                                                                   
                                 By:  /s/ TERRANCE RONAN           
                                 Name:   Terrence Ronan            
                                 Title:  Vice President            
                                                                   
                                                                   
                                 INTERNATIONALE NEDERLANDEN (U.S.) 
                                 CAPITAL CORPORATION               
                                                                   
                                 By:  /s/ CHRISTOPHER R. WAGNER    
                                 Name:   Christopher R. Wagner     
                                 Title:  Vice President             


DEN NORSKE BANK ASA              DEN NORSKE BANK ASA

By:  /s/ BYRON L. COOLEY         By:  /s/ WILLIAM V. MOYER
Name:   Byron L. Cooley          Name:   William V. Moyer
Title:  Senior Vice President    Title:  First Vice President
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                                      -4-

                                 COMERICA BANK-TEXAS


                                 By:  /s/ JAMESKIMBLE
                                 Name:   James Kimble
                                 Title:  Assistant Vice President


                                 WELLS FARGO BANK (TEXAS),
                                 NATIONAL ASSOCIATION


                                 By: /s/ ANN RHOADS
                                 Name:   Ann Rhoads
                                 Title:  Vice President
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                                      -5-

                           RATIFICATION OF GUARANTY


  The undersigned Guarantor acknowledges and accepts the foregoing Amendment and
ratifies and confirms in all respects such Guarantor's obligations under the
Guaranty dated as of August 23, 1995 (the "Guaranty") executed and delivered by
the Guarantor to the Agent and the Lenders.  The undersigned Guarantor further
agrees that references in the Guaranty to the Resources Credit Agreement shall
be references to the Third Amended and Restated Credit Agreement dated as of
April 11, 1996 among the undersigned Guarantor, ING as agent and the lenders
named therein.


PLAINS RESOURCES INC.

    /s/ MICHAEL R. PATTERSON
By:     Michael R. Patterson
Title: Vice President