<PAGE>
 
                                                                    EXHIBIT 10.N


                                           August 23, 1995



PMCT Inc.
1600 Smith Street
Houston, TX 77002

  Re:  Uncommitted Secured Demand Transactional Line of Credit Facility
       ----------------------------------------------------------------

Gentlemen:

     The purpose of the following is to outline the parameters of the
uncommitted secured demand transactional line of credit facility (the "PMCT
                                                                      -----
FACILITY") which The First National Bank of Boston ("FNBB"), Internationale
--------                                             ----
Nederlanden (U.S.) Capital Corporation ("ING") and such other banks as may from
                                         ---
time to time become parties hereto by signing below or by executing an
Instrument of Accession in the form of Exhibit A attached hereto (collectively,
the "LENDERS") and FNBB, as agent for the Lenders (in such capacity, the
     -------
"AGENT"), are prepared to provide to PMCT Inc. ("BORROWER") for the issuance of
 -----                                           --------
standby letters of credit and demand loans (each an "ACCOMMODATION" and
                                                     -------------
collectively, the "ACCOMMODATIONS"). An "uncommitted secured demand
                   --------------    
transactional line of credit facility" means that the Lenders shall have no
obligation, under any circumstances, to issue, grant or make any Accommodation.
Each request made by Borrower for an Accommodation shall be reviewed by the
Lenders on a case by case basis and the decision to grant any Accommodation
shall be made by the Lenders in their absolute and sole discretion in light of
considerations which the Lenders in their sole discretion deem pertinent and
irrespective of whether or not the Borrower is in compliance with any of the
guidelines set forth in Schedule 4 hereto. The Lenders also reserve the right to
summarily refuse any request for an Accommodation without any review as
contemplated by the preceding sentence. Accordingly, the Lenders have no
commitment to make any Accommodation available. FURTHERMORE, THE LENDERS MAY
DEMAND REPAYMENT OF ANY AMOUNTS OUTSTANDING HEREUNDER OR WITH RESPECT TO ANY
ACCOMMODATION AND, AS HEREINAFTER PROVIDED, PREPAYMENT OF ANY AND ALL UNDRAWN
AMOUNTS AVAILABLE FOR DRAWING UNDER ANY ISSUED AND OUTSTANDING L/C (AS
HEREINAFTER DEFINED) OR OF ANY OBLIGATIONS SECURED BY SUCH L/C AT ANY TIME IN
THE LENDERS' SOLE DISCRETION.

     1.   THE FACILITY.   The parameters of the uncommitted secured demand
          ------------ 
transactional line of credit facility shall be as follows until further notice
by the Agent and the Lenders:
<PAGE>
 
                                       -2-



          (a) BORROWER:  PMCT Inc., a Delaware corporation.
              --------                                     

          (b) PMCT FACILITY AMOUNT:  The aggregate amount of the Accommodations
              --------------------                                             
which may be made available by the Lenders and which will be allowed to be
outstanding at any one time under the PMCT Facility shall be restricted to the
lesser of (i) the PMCT Sublimit (as hereinafter defined) or (ii) the excess of
(x) an amount which shall be $70,000,000 during periods when the NYMEX price for
West Texas Intermediate crude oil ("WTI") is equal to or less than $20.00 per
                                    ---                                      
barrel and which shall increase by $2,500,000 for each $1.00 per barrel increase
in the NYMEX price of WTI above $20.00 per barrel up to a maximum amount of
$80,000,000, such increase in the amount of the PMCT Facility to be determined
at the time of the issuance or advance of any Accommodation over (y) the then
aggregate amount of the outstanding Accommodations made by the Lenders to or for
the account of Plains Marketing & Transportation Inc., a Delaware corporation
("MARKETING") under that certain letter agreement of even date herewith, as
-----------                                                                
amended and in effect (the "MARKETING AGREEMENT"), among the Lenders, the Agent
                            -------------------                                
and Marketing (such lesser amount, as in effect from time to time under the PMCT
Facility, hereinafter referred to as the "PMCT FACILITY AMOUNT").  PLEASE NOTE
                                          --------------------                
THAT THE LENDERS MAY MODIFY THE AMOUNT OF THE PMCT FACILITY AMOUNT AT ANY TIME
IN THEIR SOLE DISCRETION.  In the event such modification results in a decrease
in the PMCT Facility Amount below the then aggregate amount of outstanding
Accommodations, the Borrower will within one (1) Business Day (as defined below,
or as hereinafter provided, repay the amount of outstanding Accommodations in
excess of the modified PMCT Facility Amount or prepay the undrawn amounts
available for drawing under L/Cs in excess of the modified PMCT Facility Amount,
unless notified otherwise by the Agent. As used herein, "BUSINESS DAY" shall
                                                         ------------       
mean any day on which commercial lenders are not authorized or required to close
in Boston, Massachusetts or New York,, New York.

          (c) LENDERS' SHARES:  Each Lender shall be offered the opportunity to
              ---------------                                                  
fund or participate in that percentage of every requested Accommodation as set
forth opposite such Lender's name on Schedule 1 hereto (its "SCHEDULE 1
                                                             ----------
PERCENTAGE").  If any Lender shall decline to so fund or participate in such
----------                                                                  
requested Accommodation, any combination of one or more of the other Lenders may
additionally fund or purchase a participation in the requested Accommodation in
the entire or a pro rata portion of such declining Lender's Schedule 1
                --------                                              
Percentage of such Accommodation, provided that in no event shall any Lender's
                                  --------                                    
interest in any Accommodation exceed a sum representing such Lender's Schedule 1
Percentage multiplied by the PMCT Facility Amount.
<PAGE>
 
                                       -3-

          (d) TYPES OF ACCOMMODATIONS; SUBLIMITS:  The types of and limits on
              ----------------------------------                             
Accommodations which may be made available under the PMCT Facility shall be the
following:

               (i)   Standby Letters of Credit: Standby letters of credit having
                     -------------------------
          an expiration date no later than 70 days after the date of issuance,
          extension or renewal thereof issued for the account of Borrower to
          support the purchase and exchange by Borrower of crude oil from
          producers and other third parties (the "L/CS");
                                                  ----
               (ii)  Demand Loans: Demand loans which may be made by the Lenders
                     ------    
          to Borrower in an aggregate amount at any time outstanding not to
          exceed $20,000,000 less the amount of Demand Loans then outstanding
          under the Marketing Agreement (the "DEMAND LOANS"), the proceeds of
                                              ------------
          which are to be used for any of the following purposes: (x) to finance
          the purchase and physical storage of crude oil which is fully hedged
          on the NYMEX and located in the terminalling and storage facilities
          owned by Plains Terminal & Transfer Corporation in Cushing, Oklahoma
          (the "CUSHING TERMINAL") or in transit in specified pipelines approved
                ----------------
          by the Lenders and listed on Schedule 5 hereto (the "CASH AND CARRY
                                       ----------              --------------
          PIPELINES") or (y) to finance up to ninety (90%) of the value,
          ---------
          calculated on a marked-to-market basis, of inventory designated by the
          requesting Borrower as so-called "working inventory" at the Cushing
          Terminal or as linefill in Cash and Carry Pipelines.

               (iii) PMCT Sublimit: For purposes hereof, "PMCT SUBLIMIT"
                     -------------                        ------------- 
          shall mean an amount equal to the lesser of (a) five (5) times
          Tangible Liquid Net Worth of PMCT or (b) $20,000,000.  For purposes
          hereof, "TANGIBLE LIQUID NET WORTH" shall mean the excess of the total
                   -------------------------                                    
          assets of PMCT as determined in accordance with generally accepted
          accounting principles ("GAAP"), over the total liabilities of PMCT as
                                  ----                                         
          determined in accordance with GAAP, less the total book value of all
          assets of PMCT properly classified as intangible assets under GAAP;
          provided that the Tangible Liquid Net Worth of PMCT shall be deemed to
          ---------                                                             
          be zero unless at least $2,000,000 of cash, Cash Equivalents or fully
          hedged crude oil inventory valued on a marked to market basis, or any
          combination of the foregoing, is included therein; and provided,
                                                                 ---------
          further, that intercompany notes payable to PMCT by Plains Resources,
          -------                                                              
          Inc. ("RESOURCES") shall be included in the calculation of the
                 ---------                                              
          Tangible Liquid Net Worth of PMCT (1)
<PAGE>
 
                                       -4-

          only if such promissory notes are payable on demand or within one year
          from the date on which such calculation is made and (2) only to the
          extent that the principal amount thereof does not at the time of such
          calculation exceed the unused borrowing base available to support
          additional revolving credit loans under that certain Second Amended
          and Restated Credit Agreement dated as of February 11, 1994, as
          amended, among Resources, ING, as agent, and the lenders named therein
          (the "RESOURCES CREDIT AGREEMENT").  "CASH EQUIVALENTS" means
                --------------------------      ----------------       
          repurchase agreements and short term obligations issued or guaranteed
          as to principal and interest by the United States of America and
          having a maturity of not more than 12 months from the date of
          acquisition; short-term certificates of deposit issued by any bank
          organized under the laws of the United States of America or any state
          thereof if such bank has a short-term debt rating of not less than P-1
          or A-1 or their equivalent by Moody's Investor Service or Standard &
          Poor's Corporation, respectively; short-term certificates of deposit
          issued by, and so-called Eurodollar "call deposits" at, any Lender or
          any foreign subsidiary or affiliates of such Lender, if any
          investments issued by such Lender or foreign subsidiary or affiliate,
          as applicable, has a rating of not less than A or its equivalent, or
          P-1 or A-1 or their equivalent, as applicable, by Moody's Investor
          Service or Standard & Poor's Corporation, respectively; or (d)
          commercial paper or finance company paper that is rated not less than
          P-1 or A-1 or their equivalents by Moody's Investor Service or
          Standard & Poor's Corporation, respectively.

      (e) EXPIRATION: (i) No request for any Accommodation may be made after the
          ----------
first anniversary of the date hereof unless the Lenders, in their sole
discretion and without any obligation to do so, extend such date in writing.

                      (ii) All Accommodations are payable ON DEMAND. Any Lender
          may, at any time and in its sole discretion, demand payment of all
          amounts owing to the Lenders under the PMCT Facility, provided that
                                                                --------
          any Lender not then making such demand for payment with respect to
          Accommodations made by it may waive such demand with respect only to
          the Accommodations made by it. In addition, those Lenders having a pro
                                                                             ---
          rata share of the Accommodations at least equal to sixty-seven percent
          ----
          of the Accommodations (the "REQUIRED LENDERS") may at any time in
                                      ----------------
          their sole discretion demand that the Borrower prepay in cash all or
          any part or fraction of the undrawn amount available for drawing under
          any issued and outstanding L/C or the obligations secured by such L/C
          by delivery to the
<PAGE>
 
                                       -5-

          Agent.  Each Lender demanding payment of any outstanding amounts owing
          to the Lenders under the PMCT Facility or prepayment of any undrawn
          amounts available for drawing under any issued and outstanding L/C or
          of any obligations secured by such L/C shall promptly notify the
          Borrower of the reason for such demand.

               (iii) Upon the occurrence of a Bankruptcy Event, all obligations
          and liabilities of Borrower to the Lenders with respect to the
          Accommodations, including, without limitation, the obligation to
          prepay any and all amounts available for drawing under any issued and
          outstanding L/C and all other obligations and liabilities of Borrower
          to the Lenders arising under this letter agreement (the "PMCT LETTER
                                                                   -----------
          AGREEMENT") or any agreements, instruments or documents executed in
          ---------
          connection herewith (collectively with the PMCT Letter Agreement, the
          "PMCT FACILITY DOCUMENTS") shall become immediately due and payable.
           -----------------------
          "BANKRUPTCY EVENT" means any event pursuant to which Borrower makes an
           ----------------
          assignment for the benefit of creditors, or admits in writing its
          inability to pay or generally fails to pay its debts as they mature or
          become due, or petitions or applies for the appointment of a trustee
          or other custodian, liquidator or receiver of Borrower or of any
          substantial part of its assets or commences any case or other
          proceeding relating to Borrower under any bankruptcy, reorganization,
          arrangement, insolvency, readjustment of debt, dissolution or
          liquidation or similar law of any jurisdiction, now or hereafter in
          effect, or takes any action to authorize, or in furtherance of, any of
          the foregoing, or if any such petition or application shall be filed
          or any such case or other proceeding shall be commenced against
          Borrower and Borrower shall indicate its approval thereof, consent
          thereto or acquiescence therein, or a decree or order is entered
          appointing any such trustee, custodian, liquidator or receiver or
          adjudicating Borrower bankrupt or insolvent, or approving a petition
          in any such case or other proceeding, or a decree or order for relief
          is entered in respect of Borrower in an involuntary case under federal
          bankruptcy laws as now or hereafter constituted.

          (f)  PRICING: The following will apply until further notice by the
Agent:

               (i) Demand Loans: Subject to paragraph 1(f)(v) hereof, the annual
                   ------------
          rate of interest on the unpaid principal balance of Demand Loans from
          time to time outstanding shall be equal to either (i) the Base Rate
          plus five-eighths of one percent (5/8%) per annum or (ii) the
          ----
          Eurodollar Rate for
<PAGE>
 
                                       -6-

          the applicable interest period plus two percent (2%) per annum, as
                                         ----                               
          such rates are selected by Borrower pursuant to Section 1(g) hereof,
          in each case computed on the basis of a 360-day year.  Interest
          calculated by reference to the Base Rate shall be payable quarterly in
          arrears.  Interest calculated by reference to the Eurodollar Rate
          shall be payable at the end of any applicable interest period selected
          by Borrower for such Demand Loan pursuant to Section 1(g) hereof.
          "BASE RATE" means the higher of (a) the annual rate of interest
           ---------                                                     
          announced from time to time by FNBB as its "base rate" at its head
          office in Boston, Massachusetts or (b) 1/2% above the overnight
          federal funds rate from time to time in effect; provided, that such
                                                          --------           
          rate may not be the lowest rate at which funds are made available to
          customers of FNBB at such time.  "EURODOLLAR RATE" means for any
                                            ---------------               
          interest period of one, two or three months (as selected by Borrower
          pursuant to Section 1(g) hereof), the annual rate of interest (rounded
          upwards, if necessary, to the nearest 1/16th of 1%) determined by the
          Agent at or about 10:30 a.m., Boston time, two Business Days on which
          Lenders are able to conduct eurodollar transactions preceding the
          first day of such interest period, as being (a) the average of the per
          annum rates at which deposits of United States dollars are offered to
          the Agent by prime banks in any lawful recognized market selected by
          the Agent, in its sole discretion acting in good faith and in
          accordance with its usual practice, in which United States dollars are
          offered by banking institutions to each other  for delivery on the
          first day of such interest period for the number of days comprised
          therein and in an amount equal (as nearly as may be) to the amount of
          the Demand Loan to be subject to such Eurodollar Rate divided by (b)
                                                                -------       
          one minus the Reserve Rate.  The "RESERVE RATE" for any date is the
                                            ------------                     
          maximum percentage (expressed as a decimal) in effect on such date at
          which the Agent is required to maintain reserves under Regulation D of
          the Board of Governors of the Federal Reserve System against
          "EUROCURRENCY LIABILITIES".  The Reserve Rate shall be adjusted
           ------------------------                                      
          automatically on and as of the effective date of any change therein.

                (ii) Letters of Credit.
                     ----------------- 

                     (x) For the issuance of an L/C, Borrower shall pay to any
          Lender issuing such L/C (in such capacity, the "ISSUING LENDER"), for
                                                          --------------       
          the pro rata accounts of the Lenders participating in such L/C, a fee
              --------                                                         
          equal to the greater of (A) 1-1/2% per annum of the maximum drawing
          amount of such L/C calculated for the period during which such L/C is
          outstanding or (B)
<PAGE>
 
                                       -7-

          1/4% flat of the maximum drawing amount of such L/C, such fee to be
          estimated and payable upon the issuance of such L/C and any unpaid
          balance of such fee to be payable upon any drawing under, and upon the
          expiration or termination of, such L/C.

                    (y) In addition to the fees set forth in clause (x) above,
               Borrower shall pay to the Issuing Lender for its own account an
               additional issuance fee of $100 per L/C, payable on the date of
               issuance thereof, and such other fees and charges customarily
               charged by the Agent in respect of L/Cs.

               (iii) Overdue Amounts. All overdue amounts payable with respect
                     ---------------
          to the Demand Loans and reimbursement obligations in respect to drawn
          L/Cs shall bear interest payable on demand at a rate per annum equal
          to four percent above the Base Rate from the date of such nonpayment
          until paid in full and whether before or after the entry of judgment
          thereon.

               (iv) Charges to the Borrower's Accounts.  Borrower authorizes the
                    ----------------------------------                          
          Agent and the Lenders to charge Borrower's account at any of the
          Lenders for any interest and fees stated herein and due and payable by
          Borrower; provided that in the absence of a demand for payment of the
                    --------                                                   
          Accommodations or in the case of undrawn amounts available for drawing
          under any L/C or the obligations secured by such L/C, prepayment, of
          such amounts, the Agent or the Lender charging such account shall give
          Borrower prior notice of its intention to do so.

               (v) Limitation on Interest.  It is the intention of the parties
                   ----------------------                                     
          hereto that the Agent and each Lender shall conform strictly to usury
          laws applicable to it.  Accordingly, if the transactions contemplated
          hereby would be usurious as to the Agent or any Lender under
          applicable laws then, in that event, notwithstanding anything to the
          contrary in this PMCT Letter Agreement or any other PMCT Facility
          Document, it is agreed that the aggregate of all consideration which
          constitutes interest under any law applicable to the Agent or any
          Lender that is contracted for, taken, reserved, charged or received by
          such Lender under this PMCT Letter Agreement or under any other PMCT
          Facility Document shall under no circumstances exceed the maximum
          amount allowed by such applicable law, and any excess shall be
          credited by the Agent or such Lender to the principal amount of the
          Demand Loans or, if the principal amount of the Demand
<PAGE>
 
                                       -8-

          Loans shall have been or would thereby be paid in full, refunded by
          the Agent or such Lender to Borrower.

               (vi)  No pricing applicable to any particular outstanding
          Accommodation shall be changed after the making or issuance of such
          Accommodation without Borrower's written consent.  Notwithstanding the
          above, nothing  herein shall affect the Lenders' right to demand
          payment, as set forth herein, in their sole discretion, of such
          Accommodation and to charge the rate of interest applicable to overdue
          amounts thereon as set forth in Section 1(f)(iii) if such demand is
          not met.

          (g)  PROCEDURES FOR REQUESTING ACCOMMODATIONS: The following 
               ----------------------------------------
procedures will assist the Lenders in considering Borrower's requests for 
Accommodations:

               (i)  To Request a Demand Loan:  To request a Demand Loan, the
                     ------------------------
          Borrower shall, in a notice to the Agent in the form of Exhibit B
                                                                  ---------
          hereto (a "DEMAND LOAN NOTICE"), delivered no later than 11:00 a.m.
                     ------------------                                      
          Boston, Massachusetts time (x) on the Business Day on which the Demand
          Loan is requested to be made if it is to bear interest calculated by
          reference to the Base Rate and (y) three Business Days prior to the
          day on which the Demand Loan  is requested to be made if it is to bear
          interest calculated by reference to the Eurodollar Rate, specify the
          amount of such Demand Loan, whether such Demand Loan is to bear
          interest calculated by reference to the Base Rate or to the Eurodollar
          Rate and, if such Demand Loan is to bear interest calculated by
          reference to the Eurodollar Rate, the interest period to be applicable
          thereto which may be one, two or three months and shall also specify
          the Business Day on which the Demand Loan then requested is to be
          made;

               (ii)  To Request a Letter of Credit: To request an L/C, Borrower
                     -----------------------------
          shall submit to the Agent a completed letter of credit application in
          substantially the form of Exhibit C hereto or, if the Agent shall
                                    ---------                             
          designate, upon prior notice to the Borrower, any Lender other than
          FNBB to be the Issuing Lender with respect to such requested L/C, in
          the form of letter of credit application requested by such Lender.  In
          the event that any provision of the letter of credit application shall
          be inconsistent with any provision of this PMCT Letter Agreement, then
          the provisions of this PMCT Letter Agreement shall, to the extent of
          any such inconsistency, govern; and
<PAGE>
 
                                       -9-

               (iii) Representations, etc.  Any request for an Accommodation
                     --------------------                                   
          shall automatically constitute a certification that the
          representations and warranties contained in Schedule 2  hereof are
                                                      ----------           
          true and correct on the date of such request and that all guidelines
          set forth in Schedule 4 attached to this PMCT Letter Agreement on the
                       ----------                                              
          date of such request have been and are met, unless otherwise set forth
          in a writing submitted by Borrower in connection with the request for
          the corresponding Accommodation. Disclosure that Borrower has not met
          or does not meet such guidelines shall in no way affect the Lenders'
          right to demand payment or prepayment of any Accommodation or any
          obligation secured by any L/C, as set forth herein, in their sole
          discretion or to refuse to make or issue an Accommodation in their
          sole discretion.

          (h) PAYMENTS: All payments due with respect to the Accommodations,
              --------
including, without limitation, prepayments of any undrawn amounts available for
drawing under any L/C or of the obligations secured by such L/C, shall be made
to the Agent or, with respect to an L/C, the Issuing Lender, for the respective
accounts of the Lenders and the Agent, and shall be paid to the Agent or such
Issuing Lender at the address set forth on the signature page of this PMCT
Letter Agreement for notices to the Agent and such Issuing Lender or at such
other address as the Agent or Issuing Lender may from time to time designate, in
each case in immediately available funds.

          (i) ACCOMMODATIONS: All Accommodations or transactions related thereto
              --------------                                                 
must meet the following criteria in addition to any other requirements that may
be imposed by the Lenders in their sole discretion from time to time:

               (i)  Copies of all applicable written purchase and sale contracts
          for the purchase and sale of crude oil in form acceptable to the Agent
          and other relevant third party documentation must be provided to the
          Agent and the Lenders prior to the issuance or advance of any
          Accommodation securing Borrower's obligation in connection with such
          transaction;

               (ii) The obligations of the third party purchaser in any matched
          purchase and sale transaction for which an L/C is to be issued shall
          be secured in an amount at least equal to 100% of the maximum drawing
          amount of such L/C by any combination of cash, Cash Equivalents,
          accounts receivable approved by the Agent, or accounts receivable
          secured by letters of credit from acceptable financial institutions
          advised through the Agent;
<PAGE>
 
                                      -10-


               (iii) Notification of the assignment to the Issuing Lender, as
          agent for the Lenders, of the proceeds of a sale contract relating to
          a purchase and sale transaction for which an Accommodation is to be
          issued or made has been or will be, prior to the making or issuance of
          the Accommodation, accomplished by the following means:

               (A)       Borrower has notified or will, promptly, and in any
                    event prior to the making or issuance of the related
                    Accommodation, notify the account debtor on such sale
                    contract in writing, either by separate correspondence
                    and/or in Borrower's sales contract, of the assignment to
                    the Issuing Lender, as agent for the Lenders, of proceeds of
                    such sale contract and has given or will, prior to the
                    making or issuance of the Accommodation, give irrevocable
                    instructions to such account debtor to make payment on such
                    sale contract, without offset or counterclaim unless the
                    Lenders, through the Agent, have given their specific prior
                    approval for the Borrower to give effect to any net-out or
                    offset agreements, to Borrower's account at the Issuing
                    Lender, as agent for the Lenders, by wire transfer; and

               (B)       Agent shall notify such account debtor of the
                    assignment of such sale contract and the proceeds relating
                    thereto to the Issuing Lender, as agent for the Lenders, and
                    of Borrower's instruction to pay such proceeds, without
                    offset or counterclaim, unless the Lenders, through the
                    Agent, have given their specific prior approval for the
                    Borrower to give effect to any net-out or offset agreements,
                    to Borrower's account at the Issuing Lender, as agent for
                    the Lenders, by wire transfer.

               (iv) Borrower shall endeavor, and Agent shall be satisfied that
          Borrower has endeavored, to include the notification and instructions
          referred to in clause (i)(iii)(A) above in such Borrower's sales
          contracts;

               (v)  Notification of the assignment to the Issuing Lender, as
          agent for the Lenders, of the proceeds of each letter of credit
          securing a sale contract relating to a purchase and sale transaction
          for which an 
<PAGE>
 
                                      -11-

          Accommodation is to be issued or made has been or will, prior to the
          making or issuance of the Accommodation, be accomplished by the
          following means:

               (A)       Borrower has notified the issuer of such letter of
                    credit in writing of the assignment of such proceeds to the
                    Issuing Lender, as agent for the Lenders, and has given
                    irrevocable instructions to such issuing bank to pay such
                    proceeds, without offset or counterclaim unless the Lenders,
                    through the Agent, have given their specific prior approval
                    for the Borrower to give effect to any net-out or offset
                    agreements, to such Borrower's account at the Issuing
                    Lender, as agent for the Lenders; and

               (B)       Borrower or such issuer has delivered the original of
                    such letter of credit to the Issuing Lender, as agent for
                    the Lenders.

               (vi)   Funds from any account debtor with respect to a sale
          contract in a purchase and sale transaction for which an L/C was
          issued which are received in advance of payment under such L/C shall
          be maintained as cash collateral with the Issuing Lender, as agent for
          the Lenders, for such L/C and shall be paid over and delivered to
          Borrower upon expiration (without drawdown) of such L/C unless the
          Lenders have demanded payment of any Accommodations, or prepayment of
          any undrawn amounts available for drawing under any other L/C or of
          the obligations secured thereby, as permitted herein, and such demand
          has not been met;

               (vii)  Payments made to suppliers under outstanding L/Cs will be
          made through the Issuing Lender, as agent for the Lenders, by wire
          transfer with reference to the corresponding L/C to facilitate the
          reduction or cancellation of such L/C; and

               (viii) Marine cargo, transportation, and other insurance as
          appropriate, shall name the Agent or Issuing Lender as loss payee or
          co-loss payee to the extent of its interest in any purchase and sale
          transaction for which an Accommodation has been made or issued.
<PAGE>
 
                                      -12-

          (j) REPORTING AND INSPECTION REQUIREMENTS:  For so long as any
              -------------------------------------
to request any Accommodations under this PMCT Letter Agreement and to enable the
Lenders to carry out an ongoing financial review of Borrower, Borrower shall
furnish each of the Lenders with the following:

               (i)    within 45 days following the end of each month, a monthly
          unaudited balance sheet and income statement, prepared in accordance
          with GAAP, subject to year-end adjustments, together with a
          certificate regarding the financial and other guidelines set forth in
          Schedule 4 to this Marketing Letter Agreement (the "FINANCIAL AND
          ----------                                          -------------
          OTHER GUIDELINES") in the form attached as Exhibit D hereto (and
          ----------------                           ---------           
          showing calculations for the financial performance guidelines set
          forth in paragraphs (i), (ii) and (iii) of Schedule 4 hereto)
                                                     ----------        
          demonstrating that the Borrower has met, or has not met, as
          applicable, such operating guidelines, signed by the Borrower's chief
          financial officer or principal accounting officer;

               (ii) within 120 days following Borrower's fiscal year end,
          audited balance sheets and statements of income and retained earnings
          from Price Waterhouse L.L.P. or, in lieu of Price Waterhouse L.L.P.,
          an independent public accounting firm selected by Borrower and
          acceptable to the Agent, prepared in accordance with GAAP;

               (iii) on a daily basis, MERC position reports of Borrower as of
          the end of the immediately preceding Business Day and, at such other
          intervals as the Agent may request, reports of inventory held by
          Borrower;

               (iv)  as soon as possible and in any event prior to the date
          requested for the issuance of L/Cs hereunder on a monthly basis, a
          monthly scheduling forecast report listing purchases, sales and
          exchange volumes, counterparties, pricing indices and other
          information relating to crude oil purchases and exchanges for the next
          month, together with copies of purchase and sales contracts and an
          estimate of the gross profit margin and operating income for such
          month;

               (v) immediate notice of any material adverse change in the
          business of Borrower, including without limitation, any threatened or
          pending material litigation against Borrower or any event which may
          give rise to any material litigation or claim under any environmental
          law;
<PAGE>
 
                                      -13-

               (vi)   immediate notice of any material default by Resources with
          respect to any indebtedness of Resources in excess of $50,000,
          including defaults under the Resources Credit Agreement;

               (vii)  on a monthly basis, a report of the value, calculated on
          a marked-to-market basis, of inventory designated by the Borrower as
          so-called "working inventory" at the Cushing Terminal or as linefill
          in Cash and Carry Pipelines, provided that Borrower will deliver an
                                       --------                              
          additional report within two days of any downward fluctuation in the
          value of such inventory stated in the most recent monthly report by an
          amount equal to or greater than $500,000;

               (viii) if requested by any of the Lenders, Borrower will furnish
          to the requesting Lender in connection with any Accommodation
          hereunder a statement in conformity with the requirements of Federal
          Reserve Form U-1 referred to in said Regulation U; and

               (ix) such other financial or other information about Borrower as
          any Lender may request.

     In addition, at least twice per year, and more frequently if requested by
the Required Lenders, the Lenders or any of their agents or representatives
shall have the right to conduct a commercial finance examination, including the
right to examine and make copies and abstracts from the records and books of
account of, and visit the properties of, Borrower and to discuss the affairs,
finances and accounts of Borrower with any of Borrower's officers, directors
and, upon prior written notice to Borrower, independent accountants. All
expenses relating to not more than two such commercial finance examinations per
year shall be for the account of Borrower, provided Borrower will be responsible
for more than two such commercial finance examinations per year if agreed to by
the Borrower or if any demand has been made by any Lender for the payment by
Borrower of outstanding Accommodations owing to such Lenders, or by the Required
Lenders for the prepayment of any undrawn amounts available for drawing under
any L/C or of any obligations secured by such L/C, and such demand has not been
met or withdrawn.

          (k) FINANCIAL AND OTHER GUIDELINES: In determining whether to issue or
              ------------------------------                                
extend, as applicable, any Accommodation, and in no way limiting the imposition,
from time to time, of other reasonable requirements by the Lenders or the
discretionary nature
<PAGE>
 
                                      -14-

of this PMCT Facility, the Lenders will rely on Borrower's representations and
warranties given pursuant to Section 1(g)(iii) that as of the date of each
request by Borrower for an Accommodation, each of the Financial and Other
Guidelines are met and, up to the date of such request, have been met by
Borrower, unless the Lenders have otherwise been notified by Borrower.  In order
to induce the Lenders to issue or make Accommodations, Borrower hereby agrees to
make such representations and warranties pursuant to Section 1(g) hereof and
hereby covenants that it will immediately notify the Agent and the Lenders if,
at any time, such Financial and Other Guidelines are not then met by Borrower.
Whether or not the Financial and Other Guidelines are, at any time, met shall in
no way affect the Lenders' ability to demand repayment of any Demand Loans
outstanding hereunder or to demand prepayment of any undrawn amounts available
for drawing under any L/C or of the obligations secured by such L/C or the
Lenders' right to refuse to advance any Accommodations.  The Agent, the Lenders
and Borrower hereby acknowledge that the Financial and Other Guidelines are
meant to provide parameters for Borrower's operations and serve only as a basis
for Borrower to keep the Agent and Lenders apprised of the status of its
operations.

     2.   GENERAL DOCUMENTATION.  (a) Prior to Borrower's initial request for an
          ---------------------                                                
Accommodation under this PMCT Facility, Borrower shall execute and/or deliver,
or cause to be executed and/or delivered, to the Agent and the Lenders (i) a
security agreement, in form and substance of Exhibit E hereto and in any event
                                             ---------                        
sufficient to grant to the Agent a continuing, perfected, first priority lien on
all of the assets of Borrower to secure Borrower's obligations with respect to
or in connection with the Accommodations, (ii) pledges, in form and substance of
Exhibit F hereto, of all of the Borrower's deposit and margin accounts and
---------                                                                 
acknowledgments of and consents to such pledges from the financial institutions
with which such accounts are maintained, (iii) a guaranty by Resources of all of
Marketing's payment obligations with respect to or in connection with the
Marketing Facility (as defined in the Marketing Agreement), together with a
letter of credit issued by a bank acceptable to the Lenders in the face amount
of $1,000,000 to secure Resources' obligations under such guaranty, (items (i)
through (iii), collectively, the "SECURITY DOCUMENTS"), (iv) promissory notes
                                  ------------------                         
evidencing Borrower's obligations to repay the Demand Loans and any drawings
under the L/Cs (the "NOTES") in form and substance of Exhibit G hereto and (v)
                     -----                            ---------               
such other documents and instruments requested by the Agent and the Lenders,
including, without limitation those documents and instruments listed on that
certain Closing Agenda prepared in connection with the execution of this PMCT
Letter Agreement.

          (b) Notwithstanding, the foregoing, any Accommodation which the
Lenders, in their sole discretion, may make or issue shall be on such terms and
conditions as the
<PAGE>
 
                                      -15-

Lenders may require and shall be evidenced by documentation in form and
substance satisfactory to the Lenders and shall be subject thereto.  If,
however, the Lenders shall make or issue any Accommodation and the Lenders shall
not specify other terms, conditions or documentation applicable thereto, then
such Accommodation shall be subject to the terms and conditions set forth in
this PMCT Letter Agreement.

     3.   INDEMNIFICATION.  Borrower shall indemnify and hold harmless the Agent
          ---------------                                                       
and each of the Lenders and each of their respective directors, officers,
employees and agents from and against (a) any and all damages, losses,
liabilities, reasonable costs, and expenses, including, without limitation,
reasonable legal, and/or outside accounting, investment banking and other
professional fees paid or incurred by the Agent or any of the Lenders, which
shall arise out of, result from or in any way relate to any investigation,
litigation or proceeding (actual or threatened) relating to the extension of
Accommodations under this PMCT Letter Agreement, the PMCT Facility Documents,
the PMCT Facility, any Accommodation issued or made hereunder, the violation or
alleged violation of any environmental law or involving or respecting any
inventory or product sold to or by any Borrower and (b) all related reasonable
costs and expenses, including, without limitation, legal, and/or outside
accounting, investment banking and other professional fees paid or incurred by
the Agent or any of the Lenders in connection with any effort to mitigate or
defend against such damages, losses and liabilities, actual or potential, and to
otherwise protect the Agent's or any of the Lenders' interests in connection
with any matter which may give rise to such damages, losses and liabilities (but
excluding any such damages, losses, liabilities, costs and expenses, and related
costs and expenses, incurred by reason of the gross negligence or willful
misconduct by the person or entity seeking to be indemnified hereunder).
Without prejudice to any of the foregoing provisions of this Agreement, Borrower
will on demand by any Lender indemnify such Lender against any losses, costs and
expenses which such Lender may at any time or from time to time sustain or incur
with respect to any Demand Loan which bears interest calculated by reference to
the Eurodollar Rate as a consequence of Borrower's failure to borrow such Demand
Loan on the date designated for Borrower therefor or to repay such Demand Loan
on the last day of the Interest Period relating thereto or if any principal
payment thereof is made, whether before or after demand, on any day other than
the last day of the interest period relating thereto, including interest or fees
payable by such Lender to Lenders of funds obtained by it in order to maintain
such Demand Loan for such interest period.  Such losses, costs and expenses
shall be determined by such Lender in good faith on a commercially reasonable
basis.

     4.   EXPENSES.  Whether or not any Accommodations shall be made hereunder,
          --------                                                             
Borrower agrees to pay:
<PAGE>
 
                                      -16-

               (i)   the reasonable cost of (x) reproducing this PMCT Letter
          Agreement and the other PMCT Facility Documents and (y) any taxes
          (including any interest and penalties in respect thereto) or filing
          fees payable by the Agent or the Lenders (other than taxes based upon
          the Agent's or any Lender's net income) on or with respect to the
          transactions contemplated by this PMCT Letter Agreement (the Borrower
          hereby agreeing to indemnify the Agent and the Lenders with respect
          thereto);

               (ii)  the reasonable fees, expenses and disbursements of the
          Agent, the Agent's legal counsel and any experts or consultants (which
          experts or consultants may be employees of the Agent or any Lender)
          retained by the Agent incurred in connection with the preparation,
          negotiation or interpretation of this PMCT Letter Agreement and the
          other PMCT Facility Documents, each Accommodation issued or made
          hereunder, and each amendment, modification, approval, consent or
          waiver hereto or hereunder; and

               (iii) all reasonable out-of-pocket expenses (including
          reasonable attorneys' fees and costs (which attorneys may be employees
          of the Agent or such Lender)) incurred by the Agent or the Lenders in
          connection with (x) the enforcement of this PMCT Letter Agreement, the
          PMCT Facility or any of the PMCT Facility Documents against the
          Borrower or the administration thereof, (y) any so-called "work-out"
          of the Borrowers' obligations with respect to the PMCT Facility, or
          (z) any litigation, proceeding or dispute whether arising hereunder or
          otherwise, in any way related to the Agent's or the Lenders'
          relationship with the Borrower or any of its subsidiaries under the
          PMCT Facility  Documents if with respect to any litigation, proceeding
          or dispute where the Borrowers and the Lenders are adverse parties,
          the Agent and the Lenders are the prevailing parties. The provisions
          of this section shall survive payment or satisfaction of payment of
          amounts owing under or with respect to the PMCT Facility Documents.

     5.   CONSENTS, AMENDMENTS, WAIVERS, ETC.  Except as otherwise expressly
          -----------------------------------                                
provided in this PMCT Letter Agreement, any consent or approval required or
permitted by this PMCT Letter Agreement to be given by the Agent and the Lenders
may be given, and any term of this PMCT Letter Agreement or the PMCT Facility
Documents may be amended, and the performance or observance by the Borrower of
any terms of this PMCT Letter Agreement or the other PMCT Facility Documents may
be waived (either
<PAGE>
 
                                      -17-

generally or in a particular instance and either retroactively or prospectively)
with, but only with, the written consent of the Borrower, the Agent and the
Required Lenders. Neither (a) the provisions of Section 1(e), requiring that the
Required Lenders are necessary to make a demand for prepayment of any undrawn
amounts available for drawing under any L/C or of any obligations secured by
such L/C nor (b) any modification to the defined term "Required Lenders" shall
be made without the written consent of the Borrower, the Agent and all of the
Lenders.

     6.   ASSIGNMENT.  Any Lender may, with the prior written consent of the
          ----------                                                        
Borrowers, the Agent and ING, such consent not to be unreasonably withheld, make
an assignment of any portion of its share of the PMCT Facility, provided that
the assignee with respect thereto and Borrower shall execute an Instrument of
Accession in the form of Exhibit A attached hereto.
                         ---------                 

     7.   NOTICES, ETC.  Except as otherwise expressly provided herein or in the
          ------------                                                          
other PMCT Facility Documents, all notices and other communications made or
required to be given pursuant to this PMCT Letter Agreement or the other PMCT
Facility Documents to any party hereto shall be in writing and shall be
delivered in hand, mailed by United States registered or certified first class
mail, postage prepaid, sent by overnight courier, or sent by telecopy, facsimile
or telex and confirmed by delivery via courier or postal service at the address
set forth for such party on the signature pages hereto or at such other address
for notice as such party shall have last furnished in writing to the person
giving the notice.  Any such notice or demand shall be deemed to have been duly
given or made and to have become effective upon receipt thereof.

     8.   DEFINITIONS. Any capitalized term defined herein shall have such
          -----------                                                       
meaning in each place it appears herein, including in any schedules and exhibits
hereto.

     9.   MISCELLANEOUS.
          ------------- 

     With respect to Borrower, this PMCT Letter Agreement is for Borrower's
information only and Borrower is not to show this PMCT Letter Agreement to, and
it is not to be relied upon by, third parties (except that Borrower may file a
copy of, or show, or both, as applicable, this PMCT Letter Agreement to the
Securities Exchange Commission ("SEC") as required by the SEC) and this PMCT
                                 ----                                       
Letter Agreement creates no rights in any such third party.  The Agent and the
Lenders shall not disclose the terms of this PMCT Letter Agreement except in
accordance with standard and customary banking practices or as required by law
or legal practice or to assignees or potential
<PAGE>
 
                                      -18-

assignees.  This PMCT Letter Agreement constitutes the entire understanding
among Borrower, the Lenders and the Agent on this subject and supersedes all
prior discussions.

     No failure on the part of the Agent or the Lenders to exercise and no delay
in exercising, and no course of dealing with respect to any right, power, or
privilege under this PMCT Letter Agreement or any of the other PMCT Facility
Documents shall operate as a waiver thereof. Any waiver of any provision hereof
shall be effective only in the specific instance granted and shall not operate
as a continuing waiver of such provision.

     THIS LETTER AGREEMENT AND THE PMCT FACILITY DOCUMENTS AND INSTRUMENTS
EXECUTED IN CONNECTION HEREWITH SHALL FOR ALL PURPOSES BE GOVERNED BY AND
CONSTRUED IN ACCORDANCE WITH THE INTERNAL LAWS OF THE COMMONWEALTH OF
MASSACHUSETTS (WITHOUT REFERENCE TO CHOICE OF LAW PROVISIONS). THE BORROWER
AGREES THAT ANY SUIT FOR THE ENFORCEMENT OF THIS PMCT LETTER AGREEMENT OR ANY
RELATED DOCUMENT OR INSTRUMENT MAY BE BROUGHT IN THE COURTS OF THE COMMONWEALTH
OF MASSACHUSETTS OR ANY FEDERAL COURT SITTING THEREIN AND CONSENTS TO THE NON-
EXCLUSIVE JURISDICTION OF SUCH COURT AND IRREVOCABLY WAIVES TO THE FULLEST
EXTENT PERMITTED BY LAW, ANY OBJECTION WHICH IT MAY NOW OR HEREAFTER HAVE TO THE
LAYING OF THE VENUE OF ANY SUCH PROCEEDING BROUGHT IN SUCH A COURT AND ANY CLAIM
THAT ANY SUCH PROCEEDING BROUGHT IN SUCH COURT HAS BEEN BROUGHT IN AN
INCONVENIENT FORUM. BORROWER CONSENTS TO SERVICE OF PROCESS IN ANY SUCH SUIT
BEING MADE UPON BORROWER BY CERTIFIED OR REGISTERED MAIL, POSTAGE PREPAID AND
RETURN RECEIPT REQUESTED, AT THE ADDRESS FOR BORROWER SPECIFIED IN SECTION 7
HEREOF. SERVICE OF PROCESS SERVED IN THE MANNER SET FORTH IN THE PRECEDING
SENTENCE SHALL BE EFFECTIVE FIVE BUSINESS DAYS AFTER THE DATE ON WHICH SUCH
SERVICE OF PROCESS IS SENT AS PROVIDED ABOVE; PROVIDED THAT SUCH FIVE BUSINESS
                                              --------
DAY PERIOD SHALL NOT APPLY IF THE AGENT OR ANY LENDER TAKES ANY ACTION OR FILES
ANY MOTION, AND SHALL NOT PREVENT THE AGENT OR ANY LENDER FROM TAKING ANY
ACTION, OR FILING ANY MOTION, SEEKING AN INJUNCTION OR OTHER EMERGENCY ORDER OR
RELIEF WITHOUT REGARD TO THE FIVE BUSINESS DAY PERIOD PROVIDED FOR IN THIS
SENTENCE AND SUCH ACTION OR MOTION WILL NOT BE CONSIDERED TO BE EX PARTE EXCEPT
                                                                -------- 

AS OTHERWISE PROVIDED BY APPLICABLE LAW.
<PAGE>
 
                                      -19-

     EACH OF THE AGENT, THE LENDERS AND THE BORROWER HEREBY WAIVES ITS RIGHT TO
A TRIAL BY JURY IN ANY ACTION, PROCEEDING, CLAIM OR COUNTERCLAIM ARISING OUT OF
ANY DISPUTE IN CONNECTION WITH THIS LETTER AGREEMENT OR ANY OTHER PMCT FACILITY
DOCUMENT OR ANY RIGHT OR OBLIGATION HEREUNDER OR THEREUNDER. Except as
prohibited by law, each of the Agent, the Lenders and the Borrower hereby waives
any right which it may have to claim or recover in any litigation referred to in
the preceding sentence any special, exemplary, punitive consequential damages or
any damages other than, or in addition to, actual damages. Each of the Agent,
the Lenders and the Borrower (i) certifies that no party hereto nor any
representative, agent or attorney for any party hereto has represented,
expressly or otherwise, that such party would not, in the event of litigation,
seek to enforce the foregoing waivers, and (ii) acknowledges that, in entering
into the PMCT Letter Agreement and the other PMCT Facility Documents such
parties are relying upon, among other things, the waivers and certifications
contained in this (S)9.

     Agent and each Lender agrees (on behalf of itself and each of its
affiliates, directors, officers, employees, agents and representatives) to use
reasonable precautions to keep confidential, in accordance with its customary
procedures for handling confidential information of this nature and in
accordance with safe and sound banking practices, any non-public information
supplied to it by Borrower or Resources as being confidential at the time the
same is delivered to Agent or such Lender, provided that nothing herein shall
limit the disclosure of any such information (i) to the extent required by
statute, rule, regulation or judicial process, (ii) to counsel for Agent or any
Lender, (iii) to bank examiners, auditors or accountants, (iv) in connection
with any litigation to which Agent or any Lender is a party or (v) to any
assignee or participant (or prospective assignee participant) so long as such
assignee or participant (or prospective assignee or participant) first enters
into a confidentiality agreement with Agent or such Lender; and provided further
that in no event shall Agent or any Lender be required to return any materials
furnished by Borrower or Resources.

     This PMCT Letter Agreement may be executed in several counterparts, each of
which shall  be an original, and all of which shall constitute one agreement.
In proving this PMCT Letter Agreement, it shall not be necessary to produce more
than one such counterpart executed by the party to be charged.  The provisions
of this PMCT Letter Agreement are severable and if any one provision hereof
shall be held invalid or unenforceable in whole or in part in any jurisdiction,
such invalidity or unenforceability shall affect only such provision in such
jurisdiction.
<PAGE>
 
                                      -20-

  Please acknowledge your understanding of the above by signing and returning
the enclosed copy of this letter.

                             Sincerely,

                             THE FIRST NATIONAL BANK OF BOSTON,
                             Individually and as Agent


                             By:       /s/ Christopher H. Holmgren           
                                     --------------------------------
                             Name:     /s/ Christopher H. Holmgren
                                     --------------------------------
                             Title:    Vice President
                                     --------------------------------


                             Address:  100 Federal Street, 01-08-02
                                  Boston, Massachusetts 02110
                               Attention:  Christopher H. Holmgren,
                                           Vice President
                             Fax:          (617) 434-4067


                             INTERNATIONALE NEDERLANDEN    
                             (U.S.) CAPITAL CORPORATION
 
 
                             By:       /s/ Robi Artman-Hodge
                                     --------------------------------
                             Name:     Robi Artman-Hodge
                                     --------------------------------
                             Title:    Managing Director
                                     --------------------------------

                             Address:  135 East 57 Street
                                  New York, New York  10022
                               Attention: 
                             Ms. Robi Artman-Hodge
                             Managing Director
                             Fax:  (212) 593-3362
                             with a copy to:
<PAGE>
 
                                      -21-

Accepted and Agreed to:

PMCT INC.

 
 
By:         /s/ Phil Kramer
           ---------------------
Name:           Phil Kramer
           ---------------------
Title:          Vice President
           ---------------------
 
Address:      1600 Smith Street
              Houston, Texas  77002
   Attention: Mr. Phillip D. Kramer
Fax:          (713) 654-1523