Document:

<PAGE>
[***] TEXT OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND EXCHANGE
COMMISSION PURSUANT TO A CONFIDENTIAL TREATMENT REQUEST UNDER RULE 406 UNDER THE
SECURITIES ACT OF 1933, AS AMENDED AND 17 C.F.R. SECTION 200.80(b)(4)

RPC#                                                                EXHIBIT 10.6
                                 RPC AGREEMENT
                          GENERAL TERMS AND CONDITIONS

THIS AGREEMENT is made and entered into as of the 30th day of May, 2001,
("Effective Date") by and between WILLIAMS REFINING & MARKETING, LLC, a Delaware
Corporation ("Seller" or "Delivering Party"), and MAPCO EXPRESS, INC., a Texas
Corporation ("Buyer" or "Receiving Party"), sometimes referred to collectively
as "Parties" or singularly as "Party".

1.  OBLIGATIONS: Seller agrees to sell, and Buyer agrees to purchase Product up
    to the specified quantity under the terms and conditions as set forth in the
    Confirmation and herein.

2.  TERM: The term of this Agreement shall be for the period provided in the
    Confirmation. The provisions hereof shall survive termination of this
    Agreement and continue to apply to any Transactions or Confirmation entered
    into between Seller and Buyer prior to the date of termination of this
    Agreement until such time as any and all obligations related to such
    Transactions are satisfied, completed or terminated. Notwithstanding the
    foregoing, the representations, warranties, and indemnification's set forth
    in this Agreement shall survive termination of this Agreement and continue
    in effect.

3.  CONFIRMATIONS: Other than for Rack Transactions, Confirmations shall be in
    the form attached hereto as Exhibit "A" (Purchase/Sale). The Confirmation
    shall bind the Parties to the particular Transaction for the purchase or
    sale in accordance with the terms set forth in the Confirmation and herein.
    More than one Confirmation may be in effect between Parties from time to
    time and at any given time. Each Confirmation shall be treated as a separate
    and independent Agreement. The Parties acknowledge that, with respect to any
    Transaction, Williams may provide only an invoice or check remittance form
    of Confirmation, pursuant to the Section of this Agreement entitled,
    "Payment." Williams hereby adopts its check remittance invoice, or
    letterhead (including its address on each) as its signature on such
    confirmation, and its identification and authorization of such confirmation.
    For Rack Transactions, the bill of lading of the terminal operator shall
    constitute the Confirmation.

4.  PRICE: Unless otherwise provided in the Confirmation (or other writing) the
    Price shall be inclusive of all royalties, currently effective
    transportation charges, taxes, expenses and costs arising from or
    attributable to the Product prior to its delivery to the Delivery Point(s).
    If a transaction is for sale of Product on a delivered basis at Receiving
    Party's destination, the Confirmation shall so state, and any transportation
    from the Delivering Terminal to the Delivery Point(s) shall be borne by
    Receiving Party.

5.  PRODUCT: Shall mean any petroleum products which are sold or purchased in
    Transaction(s).

6.  TRANSPORTATION: Unless otherwise set forth in the Confirmation, Delivering
    Party shall be responsible for all arrangements necessary to deliver Product
    hereunder to the Delivery Point(s) and Receiving Party shall be responsible
    for all arrangements necessary to receive Product at the Delivery Point(s).

7.  TAXES AND CHARGES:

    (A) DELIVERING PARTY'S OBLIGATION: Delivering Party shall pay any and all
    Taxes, fees, or other charges (with the exception of the product excise
    taxes noted below in Section 7.C) imposed or assessed by governmental or
    regulatory bodies, with respect to the Product(s) delivered hereunder, the
    taxable incident of which occurs before the transfer of title to the
    product(s) to Receiving Party.

    (B) RECEIVING PARTY'S OBLIGATION: Receiving Party shall pay any and all
    Taxes, fees, or other charges (with the exception of the product excise
    taxes noted below in Section 7.C) imposed or assessed by governmental or
    regulatory bodies, with respect to the Product(s) delivered hereunder, the
    taxable incident of which occurs after transfer of title to the product(s)
    to Receiving Party.

    (C) PRODUCT EXCISE TAXES: Any and all Taxes, fees, or other charges imposed
    or assessed by governmental or regulatory bodies, the taxable incident of
    which is the transfer of title or the delivery of the Product(s) hereunder,
    or the receipt of payment therefor, regardless of the character, method of
    calculation or measure of the levy or assessment, shall be paid by the Party
    upon whom the Tax, fee, or charge is imposed by law, except that Receiving
    Party shall reimburse Delivering Party for all federal, state, and local
    gasoline, motor fuel, sales, use, gross receipt, and other excise taxes,
    fees, or charges that are imposed by law on Delivering Party.

    (D) EXEMPTION CERTIFICATES: Prior to the date any payment required hereunder
    becomes due, Receiving Party shall furnish to Delivering Party all current
    exemption or resale certificates or direct pay permits required or permitted
    by law for use by Receiving Party regarding the imposition or payment of any
    state or federal excise, sales or use taxes.

8.  INSURANCE: With respect to Product picked up at Seller's Facilities, for
    Seller's future protection, but without releasing, waiving or limiting the
    obligations of Buyer herein, the Buyer, its agents, contractors and any
    subcontractors (collectively referred to as "Transporter") will carry or
    cause to be carried and maintained in force throughout the entire

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     term of this agreement insurance coverage as described in paragraphs (a)
     through (d) below with insurance companies acceptable to Seller. All costs
     of such coverages and all deductible amounts will be for the sole account
     of Transporter.

          (a)  Worker's Compensation insurance complying with the laws of the
     State of States having jurisdiction over each employee, and Employer's
     Liability with limits of $1,000,000 each accident, $1,000,000 disease each
     employee and $1,000,000 disease policy limit.

          (b)  Commercial or Comprehensive General Liability insurance on an
     occurrence form with a combined single limit of $1,000,000 each occurrence,
     and annual aggregates of $1,000,000, for bodily injury and property damage,
     including coverage for blanket contractual liability, broad form property
     damage, personal injury liability, independent contractors and
     products/completed operations and sudden and accidental pollution.

          (c)  Automobile Liability insurance with a combined single limit of
     $1,000,000 each accident for bodily injury and property damage to include
     coverage for all owned, non-owned, and hired vehicles with the following
     endorsement: MCS-90 (Motor Carrier Act of 1980) and CA-9948 (Pollution
     Liability Broadened Coverage for Covered Autos) or equivalent.

          (d)  Excess Or Umbrella Liability Insurance with a combined single
     limit of $1,000,000 each occurrence, and annual aggregates of $1,000,000,
     for bodily injury and property damage covering excess of Employer's
     Liability insurance and the insurance described in (b) and (c) above.

     In each of the above described policies, Transporter agrees to waive and
     will require its insurers to waive any rights of subrogation or recovery
     they may have against Seller or its parent, subsidiary or affiliated
     companies as respects any transportation of product after title has passed
     from Seller to Buyer. Under the policies described in (b), (c) and (d)
     above, Seller and its parent, subsidiaries and affiliated companies will be
     named as additional insureds, and such insurance will serve as primary
     coverage for Seller, its parent, subsidiary and affiliated companies as
     respects any transportation of Product after title has passed from Seller
     to Buyer. Non-renewal or cancellation of policies described above, will be
     effective only after written notice is received by Seller from the
     insurance company thirty (30) days in advance of any such non-renewal or
     cancellation. Upon request, Transporter will deliver to Seller certificates
     of insurance on an Accord 25 or 25S form evidencing the existence of the
     insurance coverage required above.

9.   FINANCIAL RESPONSIBILITY: When reasonable grounds for insecurity of payment
     or title to Product arise, either party may demand in writing adequate
     assurance of performance, and in the absence of such assurance from the
     other Party within ten (10) Business Days from the date of delivery of such
     demand, suspend further performance and terminate the Agreement. Adequate
     assurance shall mean sufficient security in the form and for the term
     reasonably specified by the Party demanding assurance ("Demanding Party"),
     including, but not limited to, a standby irrevocable letter of credit, a
     prepayment, a security interest in an asset acceptable to Demanding Party
     or a performance bond or guarantee by a creditworthy entity. In the event
     either Party shall (i) make an assignment or any general arrangement for
     the benefit of creditors; (ii) file a petition or otherwise commence,
     authorize, or acquiesce in the commencement of a proceeding or cause under
     any bankruptcy or similar law for the protection of creditors or have such
     petition filed or proceeding commenced against it; (iii) otherwise become
     bankrupt or insolvent (however evidence); or (iv) be unable to pay its
     debts as they fall due; then the other Party shall have the right to either
     withhold and/or suspend deliveries or payment, net and/or set off all
     transactions outstanding between the Parties, use all rights, counterclaims
     and other defenses which it is or may be entitled to at law or arising from
     the Agreement, and/or immediately terminate the Agreement without prior
     notice. If a party to this Agreement becomes subject to Bankruptcy Code
     proceeding, it is understood and agreed that the other Party shall be
     entitled to exercise its contractual right to liquidate as a forward
     contract merchant under Section 556 of the U.S. Bankruptcy Code.

10.  DELIVERIES: Title to the Product and risk of loss shall pass from Seller to
     Buyer at the point of delivery specified hereunder.

     A.   When FOB origin, delivery shall be deemed to have been completed:

          i.   To ships or barges when the Product has passed the dock loading
               flange;

          ii.  To tank trucks when the Product has actually been delivered into
               the transport trailer;

          iii. To tank cars when the carrier accepts the same for shipment;

          iv.  To pipelines upon metering of the Product; or

          v.   To the terminal.

     B.   When FOB destination, delivery shall be deemed to have been
               completed:

          i.   From ships or barges when the Product has passed the vessel
               discharge flange;

          ii.  When Product is delivered into Buyer's tank.

          iii. From tank cars when carrier delivers same at the destination; or

          iv.  From pipeline upon metering of the Product.

11.  INSPECTION: Product delivered hereunder is subject to inspection and
     approval at the Delivery Point(s). Each Party shall be entitled to have its
     representatives present during all loadings, unloadings, tests and
     measurements involving delivery of Product under this Agreement. Each party
     may secure at its own expense outside inspectors to perform gauging,
     sampling and testing.

12.  WARRANTY OF TITLE: Seller warrants title to all Product delivered by it
     hereunder, and further warrants that it has the right to sell and transfer
     title to the same and that said Product is free and clear of all liens,
     claims and encumbrances. In the event of any adverse claim being asserted
     against the Product, Buyer may withhold payment, without interest, of sums
     due hereunder up to the amount of the claim, and, in the case of exchanges,
     Buyer may suspend its corresponding

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    delivery obligation until such claim shall have been finally determined or
    until Seller shall have furnished other adequate securities or indemnities.

13. LOADING OBLIGATIONS: Each Party making deliveries of Product hereunder shall
    comply, or cause compliance with, all federal, state and local laws,
    ordinances, rules and regulation (including but not limited to, all
    hazardous materials regulations of the U.S. Department of Transportation
    (DOT) and particularly 49 C.F.R., Part 173) as same pertain to all aspects
    of a shipper's obligations in tendering Product hereunder for
    transportation.

14. MEASUREMENT:

    a. On all deliveries into tank cars, the quantity shall be determined by
       meter or by official tank car capacity tables and tank car slip tube
       gauging device in accordance with GPA publication 8162 and all revisions
       hereof.

    b. On all deliveries into transport and tank truck equipment, quantities
       shall be determined by meter, rotary gauging device or weighing, in
       accordance with GPA publication 8162 and 8168 and all revisions hereof.

    c. On all deliveries into pipeline, quantities shall be determined by
       pipeline meter in accordance with American Petroleum Institute Manual of
       Petroleum Measurement Standards. The volume of the component products in
       E-P and raw edition of GPA publication 8182.

    d. On all deliveries to ships or barges, delivering company's shore tank
       measurements shall determine quantity, unless otherwise agreed upon.

    e. All quantities shall be corrected to 60(degrees) degrees
       Fahrenheit or as otherwise agreed.

15. PAYMENT: Unless otherwise set forth in the Confirmation, any amount due
    shall be paid by wire transfer or Automated Clearing House (ACH) within
    [***] Business Days from receipt of the invoice. If Buyer disagrees with the
    amount of any invoice for any reason, Buyer shall immediately notify Seller
    of such so that the difference may be resolved before the due date for
    payment of the invoice. If Buyer fails to give such notification or if Buyer
    and Seller do not resolve such disagreement before the date due, such
    invoice shall be paid in full according to the terms on the due date, such
    payment to be subject to adjustment upon final resolution of the
    disagreement. Furthermore, in the event of suit or other action to recover
    any monies due under this Agreement, the prevailing Party shall be entitled
    to recover its reasonable costs of collection, including court costs and
    attorneys' fees. Any amounts due Delivering Party and not received in the
    time period set forth above shall bear interest at the then effective [***]
    published under "Money Rates" by The Wall Street Journal, plus [***] percent
    [***] per annum, from the date due until the date of payment not to exceed
    the maximum rate allowed by law.

16. FAILURE TO PAY: Should Receiving Party fail to pay any amount due within the
    time period set forth above, Delivering Party may (a) offset all or any
    portion of the unpaid balance against moneys owed by Delivering Party under
    this Agreement or any other agreement between the Parties, (b) suspend
    deliveries of Product, and/or (c) after providing ten (10) Business Days'
    prior written notice during which time payment is not received by Delivering
    Party, terminate this Agreement; provided, however, that the exercise of
    such rights shall be in addition to any and all other remedies available to
    Delivering Party, whether in law or equity, under this Agreement or
    otherwise. If a Party to this Agreement becomes subject to Bankruptcy Code
    proceedings, it is understood and agreed that the other Party shall be
    entitled to exercise its contractual right to liquidate as a forward
    contract merchant under Section 556 of the Bankruptcy Code.

17. CREDIT: Receiving Party shall establish and maintain credit satisfactory to
    Delivering Party during the term of this Agreement. If Receiving Party fails
    to maintain satisfactory credit, Delivering Party may suspend deliveries of
    Product until satisfactory credit is reestablished.

18. CHOICE OF LAW: This agreement shall be governed by and construed in
    accordance with laws of Oklahoma (without regard to choice of law
    provisions) and venue shall be the State of Oklahoma, County of Tulsa.

19. RULES AND REGULATIONS: This Agreement is made subject to, and both parties
    shall comply with, all valid federal and state laws and all valid rules,
    regulations, and orders of Federal, State, or local Government authorities
    related to the sale, purchase, or exchange of Product hereunder, including
    but not limited to the Equal Employment Opportunity Laws and Executive Order
    Number 11246, which are incorporated herein by reference, and neither party
    shall be held liable for any default resulting from compliance therewith.

20. ASSIGNMENT: This Agreement shall be binding upon and shall inure to the
    benefit of the respective successors and assigns of the Parties hereto,
    provided that this Agreement shall not be transferred or assigned, by
    operation of law or otherwise, by either Party without the other Party's
    prior written consent, which consent shall not be unreasonably withheld,
    conditioned or delayed. Either Party, however, may assign its rights and
    obligations hereunder to any parent or affiliate which succeeds by
    assignment, purchase, merger, consolidation or otherwise to all or
    substantially all of the assets of the assigning party upon written notice
    to the other Party. Nothing in this clause in any way prevents either Party
    from pledging or mortgaging all or any part of such Party's property as
    security.

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21. FORCE MAJEURE: Except with regard to a Party's obligation to make payments
    as they become due hereunder, performance by either Party shall be excused
    when and to the extent that its performance under this Agreement is
    prevented, wholly or in part, by Force Majeure. The term "Force Majeure"
    means any event or occurrence beyond the reasonable control of a Party that
    prevents in whole or in part the performance by such Party of any obligation
    or condition under this Agreement, including, but not limited to acts of
    God, strikes, lockouts or other industrial disturbances (including those
    affecting persons transporting Product for either Party), acts of the public
    enemy, wars, blockades, insurrections, riots, epidemics, landslides,
    lightning, earthquakes, fires, explosions or other casualty, hurricanes,
    hurricane warnings, storms, floods, washouts, arrests and restraints of
    government (federal, state, local, civil or military) and of people, and
    civil disturbances. Neither Party shall be entitled to the benefit of Force
    Majeure under any or all of the following circumstances: to the extent that
    the failure was caused by the Party claiming suspension having failed to
    remedy the condition, and to resume the performance of such covenants or
    obligations with reasonable dispatch; the ability of either Party to obtain
    a better price for Product: the loss of markets; the loss, interruption, or
    curtailment of interruptible transportation on any Transporter necessary to
    effect receipt of delivery of Product hereunder, unless caused by a Force
    Majeure event; or economic hardship.

22. NOTICE: Notice sent by hand, facsimile, telegram, registered or certified
    mail addressed to the Party to whom such notice is given at the address of
    such Party stated herein shall be deemed sufficient upon delivery.

       Addresses for notices communications, and statements to:

     WILLIAMS/SELLER:                                  BUYER
     Mailing Address:                                  Mailing Address:
     P.O. Box 2848
     Tulsa, OK 74101
     Attn: Contract Management
     Fax: (918) 573-1935

     Street Address:                                   Street Address:
     Williams Refining & Marketing, LLC
     One Williams Center
     Tulsa, OK 74172
     Attn: Contract Management
     Invoices:                                         Invoices:
     Same as mailing
     Attn: Williams RPC Accounting
     Fax: (918) 573-1965
     Payments:                                         Payments:
     Bank One, N.A.                                    Bank:
     Account c/o Williams Refining & Marketing, LLC    Account c/o:
     Acct Number -55-79902                             Acct Number:
     ABA Number -071-0000-13                           ABA Number:
     Re: Invoice Number                                RE: Invoice Number

23. AUDIT: Each Party shall have, upon reasonable notice, the right at
    reasonable hours to examine and to copy the records of the other Party to
    the extent reasonably necessary to verify the accuracy of any invoices
    billed or quantities delivered hereunder. In the event of any inaccuracy,
    any necessary adjustments in the billing shall be promptly made; provided
    that no adjustment for any billing and payment shall be after the lapse of
    two (2) years from the date of delivery thereof.

24. NO THIRD PARTY BENEFICIARY: Nothing in this Agreement shall entitle any
    other person other than Seller or Buyer, or their successors or assigns, to
    any claim, cause of action, remedy or right of any kind relating to the
    Transaction(s) or Confirmation(s).

25. SEVERABILITY: The invalidity of one or more provisions of this agreement
    shall not affect the validity of any other provision hereof, and in case of
    any such invalidity, this Agreement shall be construed to the maximum extent
    possible as if such invalid provision(s) had not been included herein.

26. INTEGRATION: This Agreement and any Confirmation hereunder constitute the
    entire Agreement of the Parties respecting the matters addressed in such
    Confirmation and this Agreement, and supersedes all prior negotiations,
    proposals, inquiries, commitments and agreements, whether written or oral.

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<PAGE>
27. CONSTRUCTION: The Parties acknowledge and agree that the terms and
    conditions of this Agreement were freely negotiated and drafted jointly by
    the Parties, and the Parties further expressly agree that in the event of
    any ambiguity in any of the terms and conditions of this Agreement,
    including any Confirmations hereunder, such ambiguity shall not be construed
    for or against either Party on the basis that such Party did or did not
    author the same.

28. HEADINGS: The headings of the Articles, Sections and Paragraphs of this
    Agreement are for convenience of reference only and shall not constitute
    part, nor modify, define or limit any of the terms or provisions hereof.

29. COUNTERPARTS: This Agreement may be executed in counterparts, each of which
    shall be deemed an original but all of which together shall constitute one
    Agreement binding on the Parties hereto.

30. WAIVER: No waiver by either Party with respect to any breach or default of
    any right or remedy and no course of dealing, shall be deemed to constitute
    a continuing waiver of any other breach or default or of any such right or
    remedy, unless such waiver be expressed in writing signed by the Party to be
    bound.

31. TERMINAL ACTIVITY: Each Party shall be responsible for and shall indemnify
    the other Party for any loss, injury, or damage that may be caused, in whole
    or in part, by the indemnifying Party at any terminal.

32. RACK SALES: BY PULLING PRODUCT FROM THE RACK, BUYER ACKNOWLEDGES RECEIPT OF,
    AND THAT IT SHALL BE SUBJECT TO AND GOVERNED BY THE TERMS AND CONDITIONS OF
    THIS AGREEMENT WITH RESPECT TO THE PRODUCT PULLED FROM THE RACK.

33. GENERAL INDEMNITY PROVISION: Buyer shall indemnify, defend and hold Seller
    harmless from and against all loss, cost and expense, including court costs
    and reasonable attorney fees, for any claims, suits, judgments, demands,
    actions, penalties or liabilities, including injury to or death of persons,
    growing out of the operations conducted or performance hereunder by Buyer or
    arising while the Product is in Buyer's exclusive control and possession.
    Seller shall indemnify, defend and hold Buyer harmless from and against any
    loss, cost, and expense, including court costs and reasonable attorney fees,
    for any claims, suits, judgments, demands, actions, penalties or
    liabilities, including injury to or death of persons, growing out of the
    operations conducted or performance hereunder by Seller or arising while the
    Product is in Seller's exclusive control and possession. Neither Party shall
    be liable in respect of any claim to the extent same resulted from the gross
    negligence, willful misconduct or bad faith of the indemnified Party. This
    indemnity provision shall survive termination of this Agreement. Where
    personal injury, death, or loss of or damage to property is the result of
    the joint negligence or misconduct of the Parties hereto, the Parties
    expressly agree to indemnify each other and save harmless in proportion to
    their respective share of such joint negligence or misconduct.

34. REMEDIES: (A) Receiving Party's Failure to Receive: If Receiving Party fails
    to accept tender of delivery of the Contract Quantity not subject to Force
    Majeure, or if Delivering Party suspends deliveries pursuant to Section 16
    and 17 of this Agreement, Delivering Party shall sell to a third party
    purchaser, in good faith and in a commercially reasonable manner, the
    difference between the amount of Product actually accepted by Receiving
    Party and the Contract Quantity. Delivering Party shall be entitled to
    reimbursement from Receiving Party for the difference between: (1) the price
    set forth under this Agreement; and (2) any lesser price obtained from such
    third party purchaser(s) (or if Delivering Party cannot reasonably sell the
    Product that is not received by Receiving Party, a commercially reasonable
    price). Any payments due Delivering Party by Receiving Party under this
    paragraph shall be made within five (5) Business Days of receipt of
    Delivering Party's invoice for such difference; provided, however,
    Delivering Party first provides Receiving Party with information and
    documentation supporting Delivering Party's claim for reimbursement.

    (B) Delivering Party's Failure to Deliver: If Delivering Party fails to
    tender delivery of the Contract Quantity not subject to Force Majeure,
    Receiving Party shall purchase from a third party supplier, in good faith
    and in a commercially reasonable manner, the difference between the amount
    of Product actually delivered by Delivering Party and the Contract Quantity.
    Receiving Party shall be entitled to recover from Delivering Party the
    difference between: (1) the price set forth under this Agreement; and (2)
    any greater price of replacement Product purchased by Receiving Party and
    delivered to the Delivery Point(s) (or if Receiving Party cannot reasonably
    purchase the Product that is not delivered by Delivering Party, a
    commercially reasonable price). Any payments due Receiving Party by
    Delivering Party under this paragraph shall be made within five (5) Business
    Days of receipt of Receiving Party's invoice for such difference; provided,
    however, Receiving Party shall first provide Delivering Party with
    information and documentation supporting Receiving Party's claim for
    reimbursement.

35. LIMITATION OF LIABILITY: NEITHER PARTY SHALL BE LIABLE TO THE OTHER FOR
    CONSEQUENTIAL, INCIDENTAL, PUNITIVE, EXEMPLARY OR INDIRECT DAMAGES, WHETHER,
    ARISING IN TORT, CONTRACT, OR OTHERWISE EXCEPT TO THE EXTENT NECESSARY TO
    INDEMNIFY A PARTY FOR

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<PAGE>
     LIABILITY TO A THIRD PARTY. THE PARTIES INTEND THAT THE LIMITATIONS UNDER
     THIS CONTRACT IMPOSED ON REMEDIES AND THE MEASURE OF DAMAGES BE WITHOUT
     REGARD TO THE CAUSE OR CAUSES RELATED THERETO, INCLUDING, WITHOUT
     LIMITATION, THE NEGLIGENCE OR STRICT LIABILITY OF ANY PARTY, WHETHER SUCH
     NEGLIGENCE BE SOLE, JOINT OR CONCURRENT, OR ACTIVE OR PASSIVE.

36.  OUTSTANDING RACK TRANSACTIONS: Upon the effectiveness of this Agreement,
     unless otherwise agreed to in writing by the Parties to this Agreement,
     with respect to Rack Transactions, all Rack Transactions then outstanding
     between the Parties shall be subject to the terms thereof.

     In witness whereof, the Parties hereto have caused this RPC Agreement to be
executed by their respective duly authorized representatives, effective as of
the date first set forth above.

WILLIAMS REFINING & MARKETING, LLC           MAPCO EXPRESS, INC.

By: /s/   Ralph A. Hill                      By: /s/ James C. Alligood
   -------------------------------              ------------------------------

Print Name: Ralph A. Hill                    Print Name: James C. Alligood
Title: CEO & President                                  ----------------------
Date: May 31, 2001                           Title: President - CEO
                                                   ---------------------------
                                             Date:          5-31-01
                                                  ----------------------------

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<PAGE>
                                  REVISION #2
                                   EXHIBIT A

TO:      MAPCO EXPRESS, INC.
ATTN:    JAMES ALLIGOOD
DATE:    JUNE 24, 2002
FAX:
SUBJECT: PURCHASE
REF:     RPC-1638

Pursuant and subject to the terms and conditions of the RPC Agreement dated May
30, 2001, by and between Williams Refining & Marketing, LLC ("Williams") and
MAPCO EXPRESS, INC. ("Company"), this confirms the following Transaction
negotiated between Mr. John Yeager of Williams and Mr. James Alligood of MAPCO
EXPRESS, INC. on              , 2002.

BUYER:  MAPCO EXPRESS, INC.

SELLER: Williams Refining & Marketing, L.L.C.

CONTRACT TERM: JULY 1, 2002 THROUGH MAY 31, 2008; PROVIDED THAT BUYER SHALL
HAVE THE RIGHT TO TERMINATE THIS AGREEMENT UPON 365 DAYS' PRIOR WRITTEN NOTICE
OF INTENT TO TERMINATE DELIVERED TO THE SELLER.

<TABLE>
                                                       CONTRACT
PRODUCT *(SPECIAL PROVISION #6)                        QUANTITY              DIFFERENTIAL *(SPECIAL PROVISION #1)
<S>                                           <C>                            <C>
Conventional 87 octane Unleaded               [***]                          $[***]
Conventional 89 octane Unleaded               [***]                          $[***]
Conventional 93 octane Premium                [***]                          $[***]
Conventional 87 octane Unleaded, 7.8 RVP      See Special Provisions #2      $[***]
Conventional 89 octane Unleaded, 7.8 RVP      See Special Provisions #2      $[***]
Conventional 93 octane Premium, 7.8 RVP       See Special Provisions #2      $[***]
Low Sulfur Diesel Fuel                        [***]                          $[***]

</Table>

DELIVERY POINTS:  [***]
                  [***]
                  [***]

DELIVERY TRANSPORTATION TERMS:  [***]

PAYMENT TERMS:  [***]

SPECIAL PROVISIONS:

1. PRICING: DAILY RACK LIFTINGS UNDER THE TERM OF THIS AGREEMENT SHALL BE PRICED
   AT PREVIOUSLY DAY'S U.S. GULF COAST PIPELINE PROMPT PLATT'S MEAN FOR THE
   RELEVANT BASE PRODUCT WHICH WAS LIFTED PLUS THE ABOVE REFERENCED
   DIFFERENTIALS BY PRODUCT GRADE. ON WEEKENDS AND HOLIDAYS WHEN PLATT'S DOES
   NOT PUBLISH TRADING RANGES, RACK LIFTINGS WILL BE PRICED AGAINST THE MOST
   PREVIOUS PUBLISHED PLATT'S MEAN VALUE.

2.  CONTRACT VOLUMES: TOTAL MONTHLY CONTRACT LIFTINGS SHALL BE [***] OF THE
   ABOVE REFERENCED VOLUMES. ANY LIFTINGS IN EXCESS OF THIS RANGE SHALL BE
   INVOICED AT WILLIAMS REFINING & MARKETING, L.L.C.'S POSTED RACK PRICE FOR THE
   DATE OF LIFTING. IF DURING THE DURATION OF THIS AGREEMENT THE RECEIVING
   PARTY'S MONTHLY VOLUMES, ON A QUARTERLY AVERAGE DO NOT MEET MINIMUM MONTHLY
   CONTRACT VOLUME RANGE, THEN THE DELIVERING PARTY HAS THE RIGHT TO REDUCE THE
   REMAINING VOLUMES UNDER THIS AGREEMENT BY THE SAME % FOR THE DURATION OF THE
   CONTRACT. RECEIVING PARTY SHALL SUPPLY DELIVERING PARTY WITH AT LEAST THREE
   (3) WEEKS WRITTEN NOTICE OF ANY ANTICIPATED DEMAND NEEDS FOR SPECIFIC PRODUCT
   GRADE. DURING THE

[***]  CONFIDENTIAL TREATMENT REQUESTED
<PAGE>
   PERIOD OF VOC CONTROL, BUYER'S CONTRACT QUANTITIES FOR UNLEADED AND PREMIUM
   GASOLINE SHALL BE DIVIDED BETWEEN ITS 7.0 RVP AND 9.0 RVP DEMAND FOR EACH
   GRADE OF PRODUCT.

3. RATABILITY: CONTRACT QUANTITIES UNDER THE TERMS OF THIS CONTRACT SHALL BE
   LIFTED RATABLY, ON A DAILY BASIS, OVER THE COURSE OF EACH MONTH.

4. NET GALLONS: DIFFERENTIALS ARE BASED ON NET GALLONS, ADJUSTED TO 60 DEGREES
   FAHRENHEIT.

5. THE ABOVE REFERENCED DIFFERENTIALS WILL BE ADJUSTED BY ANY INCREASE OR
   DECREASE IN EITHER COLONIAL PIPELINE PUBLISHED TARIFF TO [***] FROM PASADENA,
   TX, TERMINAL HANDLING COSTS (INCLUDING IVD ADDITIVES) AND OR BY ANY CHANGES
   IN THE COLONIAL PIPELINE LINE LOSS CHARGE.

6. IF DURING THE TERM OF THIS AGREEMENT ANY STATE OR FEDERAL GOVERNMENT
   REGULATIONS REQUIRE A DIFFERENT PRODUCT SPECIFICATION TO BE DELIVERED TO THE
   CUSTOMER, THEN THE PLATT'S PROMPT PIPELINE MEAN POSTING USED FOR THE PRICE
   CALCULATIONS WILL BE CHANGED TO THE NEW REQUIRED PRODUCT(S). IF PLATT'S FAILS
   TO PUBLISH A PRICE FOR THE NEW PRODUCT(S) THEN BOTH PARTIES WILL NEGOTIATE
   THE PRICING IN ACCORDANCE TO INDUSTRY PRACTICE.

7. ALL VOLUMES OF PRODUCT SUPPLIED HEREUNDER ARE INTENDED FOR C-STORE USE ONLY
   AND SHALL NOT BE RE-SOLD TO OTHER RETAIL OR WHOLESALE MARKETERS.

These specific terms and conditions together with the RPC Agreement shall
constitute the entirety of the agreement between buyer and seller unless
COMPANY furnishes to Williams notice of alleged errors by telecopy, other
electronic transmission, or first class mail before 4:00 p.m. CST of the fifth
(5th) Business Day following the Business Day of receipt of the Confirmation
from Williams, the Confirmation shall be conclusive evidence of the Transaction
that is the subject matter thereof, and shall, along with the terms herein, be
the final expression of all its terms, notwithstanding any failure of COMPANY
to execute such confirmation.

WILLIAMS REFINING & MARKETING, LLC      MAPCO EXPRESS, INC.

By: /s/ Ted Brennan                     By: /s/ James C. Alligood
Trader:                                 Trader:
Title:                                  Title:

Prepared by:________________, Risk Control Management
Phone:_______________________
Fax No.:_____________________

[***] CONFIDENTIAL TREATMENT REQUESTED
<PAGE>
                                  REVISION #2
                                   EXHIBIT A

TO:       MAPCO EXPRESS, INC.
ATTN:     James Alligood
DATE:     June 24, 2002
FAX:
SUBJECT:  PURCHASE
REF:      RPC-1638

Pursuant and subject to the terms and conditions of the RPC Agreement dated May
30, 2001, by and between Williams Refining & Marketing, LLC ("Williams") and
MAPCO EXPRESS, INC., ("Company"), this confirms the following Transaction
negotiated between Mr. John Yeager of Williams and Mr. James Alligood of MAPCO
EXPRESS, INC. on                , 2002.

BUYER:          MAPCO EXPRESS, INC.

SELLER:         WILLIAMS REFINING & MARKETING, LLC

CONTRACT TERM:  JUNE 1, 2002 THROUGH MAY 31, 2008; PROVIDED THAT BUYER SHALL
HAVE THE RIGHT TO TERMINATE THIS AGREEMENT UPON 365 DAYS' PRIOR WRITTEN NOTICE
OF INTENT TO TERMINATE DELIVERED TO THE SELLER.

<Table>
<Caption>
                                                     CONTRACT
PRODUCT *(SPECIAL PROVISION #6)                      QUANTITY              DIFFERENTIAL *(SPECIAL PROVISION #1)
-------------------------------             --------------------------     ------------------------------------
<S>                                         <C>                                          <C>
Conventional 87 octane Unleaded             [***]                                        [***]
Conventional 89 octane Unleaded             [***]                                        [***]
Conventional 93 octane Premium              [***]                                        [***]
Conventional 87 octane Unleaded, 7.8 RVP    See Special Provisions #2                    [***]
Conventional 89 octane Unleaded, 7.8 RVP    See Special Provisions #2                    [***]
Conventional 93 octane Premium, 7.8 RVP     See Special Provisions #2                    [***]
Low Sulfur Diesel Fuel                      [***]                                        [***]
</Table>

DELIVERY POINTS:        [***]

DELIVERY TRANSPORTATION TERMS:  [***]

PAYMENT TERMS:          [***]

SPECIAL PROVISIONS:

1.  Pricing:  Daily rack liftings under the term of this agreement shall be
    priced at previous day's U.S. Gulf Coast Pipeline Prompt Platt's mean for
    the relevant base product which was lifted plus the above referenced
    differentials by product grade. On Weekends and holidays when Platt's does
    not publish trading ranges, rack liftings will be priced against the most
    previous published Platt's mean value.

2.  Contract Volumes:  Total monthly contract liftings shall be [***] of the
    above referenced volumes. Any liftings in excess of this range shall be
    invoiced at Williams Refining & Marketing, L.L.C.'s posted rack price for
    the date of lifting. If during the duration of this agreement the receiving
    party's monthly volumes, on a quarterly average do not meet minimum monthly
    contract volume range, then the delivering party has the right to reduce the
    remaining volumes under this agreement by the same % for the duration of the
    contract. Receiving party shall supply delivering party with at least three
    (3) weeks written notice of any anticipated demand needs for specific
    product grade. During the period of VOC control, buyer's contract quantities
    for unleaded and premium gasoline shall be divided between its 7.0 RVP and
    9.0RVP demand for each grade of product.

[***] CONFIDENTIAL TREATMENT REQUESTED
<PAGE>
3.  Ratability: Contract quantities under the terms of this contract shall be
    lifted ratably, on a daily basis, over the course of each month.

4.  Net Gallons: Differentials are based on net gallons, adjusted to 60 degrees
    Fahrenheit.

5.  If during the terms of this Agreement any State or Federal government
    regulations require a different product specification to be delivered to the
    customer, then the Platt's Prompt Pipeline mean posting used for the price
    calculations will be changed to the new required product(s). If Platt's
    fails to publish a price for the new product(s) then both parties will
    negotiate the pricing in accordance to industry practice.

6.  All volumes of product supplied hereunder are intended for C-Store use only
    and shall not be re-sold to other retail or wholesale marketers.

7.  The above-referenced Differentials for all gasoline (including IVD
    additives) and low sulfur diesel products listed above, other than the 93
    octane Premiums, will be adjusted during the term of this agreement to equal
    the lowest of Williams' exchange differentials (including IVD additives) in
    effect from time to time for such non-Premium gasoline and low sulfur diesel
    products delivered into the [***] (the "ADJUSTED DIFFERENTIALS"). The
    Differentials for the two 93 octane Premium gasolines listed above shall be
    adjusted during the term hereof to equal the then-current Adjusted
    Differential plus [***] basis points.

These specific terms and conditions together with the RPC Agreement shall
constitute the entirety of the agreement between buyer and seller unless COMPANY
furnishes to Williams notice of alleged errors by telecopy, other electronic
transmissions, or first class mail before 4:00 p.m. CST of the fifth (5th)
Business Day following the Business Day of receipt of the Confirmation from
Williams, the Confirmation shall be conclusive evidence of the Transaction that
is the subject matter thereof, and shall, along with the terms herein, be the
final expression of all its terms, notwithstanding any failure of COMPANY to
execute such confirmation.

WILLIAMS REFINING & MARKETING, LLC               MAPCO EXPRESS, INC.

By:     /s/Ted Brennan                           By:    /s/James C. Alligood
Trader:                                          Trader:
Title:                                           Title:

Prepared by: ______________________________, Risk Control Management

Phone: ____________________________________

Fax No.: __________________________________

[***] CONFIDENTIAL TREATMENT REQUESTED

<PAGE>
                                  REVISION #2
                                   EXHIBIT A

TO:      MAPCO EXPRESS, INC.
ATTN:    JAMES ALLIGOOD
FAX:
DATE:    JUNE 24, 2002
SUBJECT: PURCHASE
REF:     RPC-1638

Pursuant and subject to the terms and conditions of the RPC Agreement dated May
30, 2001, by and between Williams Refining & Marketing, LLC ("Williams") and
MAPCO EXPRESS, INC., ("Company"), this confirms the following Transaction
negotiated between Mr. John Yeager of Williams and Mr. James Alligood of MAPCO
EXPRESS, INC. on ________, 2001.

BUYER:        MAPCO EXPRESS, INC.

SELLER:       WILLIAMS Refining & Marketing, LLC

CONTRACT TERM: JULY 1, 2002 THROUGH MAY 31, 2008; PROVIDED THAT BUYER SHALL HAVE
THE RIGHT TO TERMINATE THIS AGREEMENT UPON 365 DAYS' PRIOR WRITTEN NOTICE OF
INTENT TO TERMINATE DELIVERED TO THE SELLER.

<TABLE>
                                    CONTRACT
PRODUCT *(SPECIAL PROVISION #6)     QUANTITY    DIFFERENTIAL *(SPECIAL PROVISION #1)
-------------------------------     --------    ------------------------------------
<S>                                 <C>         <C>
Conventional 87 octane Unleaded        [***]    [***]
Conventional 89 octane Unleaded        [***]    [***]
Conventional 93 octane Premium         [***]    [***]
Low Sulfur Diesel                      [***]    [***]
</TABLE>

DELIVERY POINTS:    [***]

DELIVERY TRANSPORTATION TERMS:     [***]

PAYMENT TERMS:      [***]

SPECIAL PROVISIONS:

1.    PRICING: DAILY RACK LIFTINGS UNDER THE TERM OF THIS AGREEMENT SHALL BE
      PRICED AT PREVIOUS DAY'S U.S. GULF COAST PIPELINE PROMPT PLATT'S MEAN FOR
      THE RELEVANT BASE PRODUCT WHICH WAS LIFTED PLUS THE ABOVE REFERENCED
      DIFFERENTIALS BY PRODUCT GRADE. ON WEEKENDS AND HOLIDAYS WHEN PLATT'S DOES
      NOT PUBLISH TRADING RANGES, RACK LIFTINGS WILL BE PRICED AGAINST THE MOST
      PREVIOUS PUBLISHED PLATT'S MEAN VALUE.

2.    CONTRACT VOLUMES: TOTAL MONTHLY CONTRACT LIFTINGS SHALL BE [***] OF THE
      ABOVE REFERENCED VOLUMES. ANY LIFTINGS IN EXCESS OF THIS RANGE SHALL BE
      INVOICED AT WILLIAMS REFINING & MARKETING, L.L.C.'S POSTED RACK PRICE FOR
      THE DATE OF LIFTING. IF DURING THE DURATION OF THIS AGREEMENT THE
      RECEIVING PARTY'S MONTHLY VOLUMES, ON A QUARTERLY AVERAGE DO NOT MEET
      MINIMUM MONTHLY CONTRACT VOLUME RANGE, THEN THE DELIVERING PARTY HAS THE
      RIGHT TO REDUCE THE REMAINING VOLUMES UNDER THIS AGREEMENT BY THE SAME %
      FOR THE DURATION OF THE CONTRACT. RECEIVING PARTY SHALL SUPPLY DELIVERING
      PARTY WITH AT LEAST THREE (3) WEEKS WRITTEN NOTICE OF ANY ANTICIPATED
      DEMAND FOR NEEDS FOR SPECIFIC PRODUCT GRADE. DURING THE PERIOD OF VOC
      CONTROL, BUYER'S CONTRACT QUANTITIES FOR UNLEADED AND PREMIUM GASOLINE
      SHALL BE DIVIDED BETWEEN ITS 7.0 RVP AND 9.0RVP DEMAND FOR EACH GRADE OF
      PRODUCT.

3.    RATABILITY: CONTRACT QUANTITIES UNDER THE TERMS OF THIS CONTRACT SHALL BE
      LIFTED RATABLY, ON A DAILY BASIS, OVER THE COURSE OF EACH MONTH.

[***] CONFIDENTIAL TREATMENT REQUESTED

<PAGE>
4.  NET GALLONS: DIFFERENTIALS ARE BASED ON NET GALLONS, ADJUSTED TO 60 DEGREES
    FAHRENHEIT.

5.  THE ABOVE REFERENCED DIFFERENTIALS WILL BE ADJUSTED BY ANY INCREASE OR
    DECREASES IN EITHER COLONIAL PIPELINE PUBLISHED TARIFF TO [***] FROM
    PASADENA, TX, TERMINAL HANDLING COSTS (INCLUDING IVD ADDITIVES) AND OR BY
    ANY CHANGES IN THE COLONIAL PIPELINE LINE LOSS CHARGE.

6.  IF DURING THE TERM OF THIS AGREEMENT ANY STATE OR FEDERAL GOVERNMENT
    REGULATIONS REQUIRE A DIFFERENT PRODUCT SPECIFICATION TO BE DELIVERED TO THE
    CUSTOMER, THEN THE PLATT'S PROMPT PIPELINE MEAN POSTING USED FOR THE PRICE
    CALCULATIONS WILL BE CHANGED TO THE NEW REQUIRED PRODUCTS(S). IF PLATT'S
    FAILS TO PUBLISH A PRICE FOR THE NEW PRODUCT(S) THEN BOTH PARTIES WILL
    NEGOTIATE THE PRICING IN ACCORDANCE TO INDUSTRY PRACTICE.

7.  ALL VOLUMES OF PRODUCT SUPPLIED HEREUNDER ARE INTENDED FOR C-STORE USE ONLY
    AND SHALL NOT BE RE-SOLD TO OTHER RETAIL OR WHOLESALE MARKETERS.

These specific terms and conditions together with the RPC Agreement shall
constitute the entirety of the agreement between buyer and seller unless COMPANY
furnishes to Williams notice of alleged errors by telecopy, other electronic
transmission, or first class mail before 4:00 p.m. CST of the fifth (5th)
Business Day following the Business Day of receipt of the Confirmation from
Williams, the Confirmation shall be conclusive evidence of the Transaction that
is the subject matter thereof, and shall, along with the terms herein, be the
final expression of all its terms, notwithstanding any failure of COMPANY to
execute such confirmation.

WILLIAMS REFINING & MARKETING, LLC                        MAPCO EXPRESS, INC.

By: /s/ Ted Brennan                                   By: /s/ James C. Alligood
Trader:                                               Trader:
Title:                                                Title:

Prepared by: ____________________________, Risk Control Management
Phone: __________________________________
Fax No.: ________________________________

[***] CONFIDENTIAL TREATMENT REQUESTED

<PAGE>
                                  REVISION #2
                                   EXHIBIT A

To:       MAPCO EXPRESS, INC.
Attn:     James Alligood
Date:     June 24, 2002
Fax:
Subject:  Purchase
Ref:      RPC-1638

Pursuant and subject to the terms and conditions of the RPC Agreement dated May
30, 2001, by and between Williams Refining & Marketing, LLC ("Williams") and
MAPCO EXPRESS, INC., ("Company"), this confirms the following Transaction
negotiated between Mr. John Yeager of Williams and Mr. James Alligood of MAPCO
EXPRESS, INC. on _______________, 2002.

Buyer:         MAPCO EXPRESS, INC

Seller:        WILLIAMS REFINING & MARKETING, LLC

CONTRACT TERM: JULY 1, 2002 THROUGH MAY 31, 2008; PROVIDED THAT BUYER SHALL
HAVE THE RIGHT TO TERMINATE THIS AGREEMENT UPON 365 DAYS' PRIOR WRITTEN NOTICE
OF INTENT TO TERMINATE DELIVERED TO THE SELLER.

<TABLE>
<S>                                <C>                <C>
                                   CONTRACT
PRODUCT *(SPECIAL PROVISION #6)    QUANTITY           DIFFERENTIAL *(SPECIAL PROVISION #1)
-------------------------------    --------           ------------------------------------
Conventional 87 octane Unleaded    [***]              [***]
Conventional 89 octane Unleaded    [***]              [***]
Conventional 93 octane PREMIUM     [***]              [***]
Low Sulfur Diesel                  [***]              [***]

</TABLE>

DELIVERY POINTS:         [***]

DELIVERY TRANSPORTATION TERMS: [***]

PAYMENT TERMS:           [***]

SPECIAL PROVISIONS:

1.   Pricing: Daily rack liftings under the term of this agreement shall be
     priced at previous day's U.S. Gulf Coast Pipeline Prompt Platt's mean for
     the relevant base product which was lifted plus the above referenced
     differentials by product grade. On Weekends and holidays when Platt's does
     not publish trading ranges, rack liftings will be priced against the most
     previous published Platt's mean value.

2.   Contract Volumes: Total monthly contract liftings shall be [***] of the
     above referenced volumes. Any liftings in excess of this range shall be
     invoiced at Williams Refining & Marketing, L.L.C.'s posted rack price for
     the date of lifting. If during the duration of this agreement the receiving
     party's monthly volumes, on a quarterly average do not meet minimum monthly
     contract volume range, then the delivering party has the right to reduce
     the remaining volumes under this agreement by the same % for the duration
     of the contract. Receiving party shall supply delivering party with at
     least three (3) weeks written notice of any anticipated demand needs for
     specific product grade. During the period of VOC control, buyer's contract
     quantities for unleaded and premium gasoline shall be divided between its
     7.0 RVP and 9.0RVP demand for each grade of product.

3.   Ratability: Contract quantities under the terms of this contract shall be
     lifted ratably, on a daily basis, over the course of each month.

[***] CONFIDENTIAL TREATMENT REQUESTED
<PAGE>
4.   NET GALLONS: DIFFERENTIALS ARE BASED ON NET GALLONS, ADJUSTED TO 60
     DEGREES FAHRENHEIT.

5.   THE ABOVE REFERENCED DIFFERENTIALS WILL BE ADJUSTED BY ANY INCREASE OR
     DECREASES IN EITHER TEPPCO PIPELINE PUBLISHED TARIFF TO [***] FROM HOUSTON
     (PASADENA), TX, TERMINAL HANDLING COSTS (INCLUDING IVD ADDITIVES) AND OR BY
     ANY CHANGES IN THE TEPPCO PIPELINE LINE LOSS CHARGE.

6.   IF DURING THE TERM OF THIS AGREEMENT ANY STATE OR FEDERAL GOVERNMENT
     REGULATIONS REQUIRE A DIFFERENT PRODUCT SPECIFICATION TO BE DELIVERED TO
     THE CUSTOMER, THEN THE PLATT'S PROMPT PIPELINE MEAN POSTING USED FOR THE
     PRICE CALCULATIONS WILL BE CHANGED TO THE NEW REQUIRED PRODUCTS(S). IF
     PLATT'S FAILS TO PUBLISH A PRICE FOR THE NEW PRODUCT(S) THEN BOTH PARTIES
     WILL NEGOTIATE THE PRICING IN ACCORDANCE TO INDUSTRY PRACTICE.

7.   ALL VOLUMES OF PRODUCT SUPPLIED HEREUNDER ARE INTENDED FOR C-STORE USE ONLY
     AND SHALL NOT BE RE-SOLD TO OTHER RETAIL OR WHOLESALE MARKETERS.

These specific terms and conditions together with the RPC Agreement shall
constitute the entirety of the agreement between buyer and seller unless
COMPANY furnishes to Williams notice of alleged errors by telecopy, other
electronic transmission, or first class mail before 4:00 p.m. CST of the fifth
(5th) Business Day following the Business Day of receipt of the Confirmation
from Williams, the Confirmation shall be conclusive evidence of the Transaction
that is the subject matter thereof, and shall, along with the terms herein, be
the final expression of all its terms, notwithstanding any failure of COMPANY
to execute such confirmation.

WILLIAMS REFINING & MARKETING, LLC                 MAPCO EXPRESS, INC.

By:    /s/Ted Brennan                              By:    /s/James C. Alligood
Trader:                                            Trader:
Title:                                             Title:

Prepared by:_______________, Risk Control Management
Phone:______________________
Fax No.:____________________

[***] CONFIDENTIAL TREATMENT REQUESTED<PAGE>
                                                                 EXHIBIT 10.6(a)

                       ASSIGNMENT AND ASSUMPTION AGREEMENT
                         (WRM, WGM, WMT, & WPSS TO PRG)

     THIS ASSIGNMENT AND ASSUMPTION AGREEMENT ("Assignment") effective as of
March 3, 2003 (the "Effective Date") is entered into by and between WILLIAMS
REFININING & MARKETING, L.L.C., WILLIAMS GENERATING MEMPHIS, L.L.C., WILLIAMS
MEMPHIS TERMINAL, INC, and WILLIAMS PETROLEUM PIPELINE SYSTEMS, INC.
(collectively the "Assignors") and THE PREMCOR REFINING GROUP INC. (Assignee").

                                   WITNESSETH:

     WHEREAS, the parties have entered in to that certain Asset Purchase and
Sale Agreement dated as of November 25, 2002, as amended (the "Purchase
Agreement"), by and among Williams Refining & Marketing, L.L.C., Williams
Generating Memphis, L.L.C., Williams Memphis Terminal, Inc., Williams Petroleum
Pipeline Systems, Inc. and Williams Mid-South Pipelines, LLC as Sellers, and The
Williams Companies, Inc., as Sellers' Guarantor (collectively, the "Seller
Parties"), and Assignee as Purchaser and Premcor Inc., as Purchaser's Guarantor
(collectively, the "Purchaser Parties"); and

     WHEREAS, Assignors, as of the Effective Date, are conveying to Assignee
and/ or its affiliate their interest in its refinery and terminal facilities in
Memphis, Tennessee, the terminal facility in West Memphis, Arkansas and the
products and crude oil pipelines that tie to the refinery and concurrently
therewith, Assignors are also conveying to Assignee the Contracts as defined in
the Purchase Agreement; and

     WHEREAS, pursuant to Purchase Agreement, Assignor wishes to assign all of
its right, title and interest in the Contracts to Assignee, and Assignee wishes
to accept and assume the same on the terms and conditions set forth herein;

     NOW, THEREFORE, for and in consideration of the premises and Ten Dollars
($10.00) in hand paid by each party to the other, and other good and valuable
consideration, the receipt and sufficiency of which is hereby acknowledged,
Assignors and Assignee hereby agree as follows:

     1. Assignment. Assignors hereby sell and assign to Assignee all of
Assignors' right, title and interest in, to and under the Contracts, except
those Contracts assigned by Assignors to The Premcor Pipeline Co and/or Assignee
by separate conveyance as of the date hereof.

     2. Assumption. Assignee hereby assumes the obligation of the Assignors
under the Contracts and agrees to faithfully perform all of the covenants,
stipulations and agreements contained therein from the date of this Assignment
and Assumption Agreement forward.

<PAGE>

     3. Assignors to Remain Liable. Notwithstanding the assignment by the
Assignors of the rights and obligations to Assignee as set forth herein,
Assignors shall remain responsible for the performance of any obligations under
the Contracts which have accrued through the date of this Assignment and
Assumption Agreement.

     4. Warranties of Assignor. This Assignment and Assumption Agreement is made
subject to, and without modification or amendment of, the Purchase Agreement.
Such representations and warranties therein as are expressly applicable to the
matters assigned by this Assignment and Assumption Agreement shall apply with
respect thereto, in accordance with the terms of the Purchase Agreement.

     5. No Modification. This Assignment and Assumption Agreement shall not be
modified, in any way whatsoever, other than in a writing signed by the parties
hereto.

                         [SIGNATURES ON FOLLOWING PAGE]

                                       2

<PAGE>

     IN WITNESS WHEREOF, Assignors and Assignee have each, individually or by
their respective authorized agent(s), executed this Assignment and Assumption
Agreement as of the date first above written.

                                        ASSIGNORS

                                        WILLIAMS REFINING & MARKETING, L.L.C.

                                        By: /s/ Randy M. Newcomer
                                            ------------------------------------
                                            Randy M. Newcomer, President

                                        WILLIAMS GENERATING MEMPHIS, L.C.C.

                                        By: /s/ Randy M. Newcomer
                                            ------------------------------------
                                            Randy M. Newcomer, President

                                        WILLIAMS MEMPHIS TERMINAL, INC.

                                        By: /s/ Randy M. Newcomer
                                            ------------------------------------
                                            Randy M. Newcomer, President

                                        WILLIAMS PETROLEUM PIPELINE SYSTEMS,
                                        INC.

                                        By: /s/ Randy M. Newcomer
                                            ------------------------------------
                                            Randy M. Newcomer, Vice-President

                                        3

<PAGE>

                                        ASSIGNEE:

                                        THE PREMCOR REFINING GROUP INC.

                                        By: /s/ Michael D. Gayda
                                            ------------------------------------
                                        Print Name: Michael D. Gayda
                                        Title: Sr. Vice President

                                        4

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