<PAGE>
                                                                 EXHIBIT 10(s)


 
                          Dated as of August 7, 1997


PMCT Inc.
1600 Smith Street
Houston, TX 77002

     Re:  Amendment No. 5 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 "PMCT 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 PMCT Inc.
(the "Borrower").  All capitalized terms used herein without definition which
are defined in the PMCT Letter Agreement shall have the same meaning herein as
therein.

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

     1.   DEMAND LOAN SUBLIMIT.  Section 1(d)(ii) of the PMCT Letter Agreement
is hereby amended by (1) deleting the amount "$20,000,000" from the first
sentence thereof and substituting the amount "$25,000,000" therefor and (2)
deleting the phrase "ninety (90%)" from clause (y) of the first sentence and
substituting the phrase "eighty percent (80%)" therefor.

     2.   PMCT SUBLIMIT.  Section 1(d)(iii) of the PMCT Letter Agreement is
hereby amended by deleting the amount "$20,000,000" from the first sentence
thereof and substituting the amount "$25,000,000" therefor.
<PAGE>
 
                                      -2-
     3.   PRICING.


     (a)  Clause (i) of the first sentence of Section 1(f)(i) of the PMCT Letter
Agreement is hereby amended by deleting the phrase "plus five-eighths of one
percent (5/8%) per annum".

     (b)  Clause (ii) of the first sentence of Section 1(f)(i) of the PMCT
Letter Agreement is hereby amended by deleting the phrase "two percent (2%)"
therefrom and substituting the phrase "one and one-half of one percent (1-1/2%)"
therefor.

     (c)  Section 1(f)(ii)(x) of the PMCT Letter Agreement is hereby amended by
deleting the percentage "1-1/2%" from clause (A) thereof and substituting "1-
1/8%" therefor and by deleting the percentage "1/4%" from clause (B) thereof and
substituting "0.1875%" therefor.

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

     (a)  paragraph (v)(C) of Schedule 4 is amended by deleting the text thereof
in its entirety and substituting the following therefor:

     "(C) The aggregate of (a) crude oil pipeline inventory and (b) crude oil
          inventory in the Cushing Terminal shall not exceed, in the aggregate
          for both PMCT Inc. and Marketing, a maximum of 300,000 barrels.
          Notwithstanding the above, inventories in crude oil pipelines shall be
          limited to 225,000 barrels."

     (b)  paragraph (v)(D) of Schedule 4 is amended by deleting the text thereof
in its entirety and substituting the following therefor:

     "(D) Inventory positions (other than fully hedged cash-and-carry positions
          which qualify under subsection (E) below) and positions which do not
          otherwise qualify under subsection (E) below shall be limited to fixed
          price time spreads for periods up to a maximum of twenty-four months
          for up to a maximum of 500,000 barrels, in the aggregate for both PMCT
          Inc. and Marketing, of crude oil hedged on the NYMEX."

     (c)  the following new paragraph (E) is inserted immediately following
paragraph (v)(D) of Schedule 4:
<PAGE>
 
                                      -3-


     "(E) Cash-and-carry barrels shall be hedged on the NYMEX for delivery
          within the next 12 months."

     (d)  the first sentence of paragraph (xi) of Schedule 4 is amended by
inserting, immediately following the phrase "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 principal amount of $150,000,000" and before the period at the end
thereof, the phrase "and that certain Indenture dated as of July 21, 1997 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 C and Series D, in the aggregate principal
amount of $50,000,000".

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

     6.   CONDITIONS PRECEDENT.  This Amendment shall become effective upon
receipt by the Agent of the following:

     (a)  a counterpart of this Amendment duly signed where indicated below by
each Lender, the Agent and the Borrower;

     (b)  a counterpart of Amendment No. 5 to the Marketing Agreement duly
signed where indicated by Marketing, each Lender, the Agent and Resources;
<PAGE>
 
                                      -4-

     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 6 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.

                              PMCT INC.


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


                              BANKBOSTON, N.A.,
                              Individually and as Agent

                              By:  /s/ CHRISTOPHER C. HOLMGREEN
                              Name:   Christopher C. Holmgreen
                              Title:  Director


                              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/ WILLIAM C. MOYER      By: /s/ BYRON L. COOLEY
Name:   Willliam C. Moyer     Name:   Byron L. Cooley
Title:  First Vice President  Title:  Senior Vice President
<PAGE>
 
                                      -5-

                              COMERICA BANK-TEXAS


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


                              WELLS FARGO BANK (TEXAS),
                              NATIONAL ASSOCIATION


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