September 23, 2002

TO THE LENDERS PARTY TO THE REVOLVING
CREDIT AGREEMENT REFERRED TO BELOW

      Re:    Sixth Amendment to Guaranty

Ladies and Gentlemen:

        We refer to (1) the Revolving Credit Agreement dated as of November 16,
1999 among Frontier Oil and Refining Company (the “Borrower”), the lenders
referred to therein (the “Lenders”), Union Bank of California, N.A., as
Administrative Agent (the “Agent”), Documentation Agent and Lead Arranger, and
BNP Paribas (successor to Paribas), as Syndication Agent and Lead Arranger, as
amended by the First Amendment to Revolving Credit Agreement and First Amendment
to Guaranty dated September 20, 2000, the Second Amendment to Revolving Credit
Agreement and Second Amendment to Guaranty and First Amendment to Clawback
Agreement dated as of June 20, 2001 and the Third Amendment to Revolving Credit
Agreement and Notice Thereunder and Fifth Amendment to Guaranty dated January
30, 2002 (said Agreement, as so amended, herein called the “Credit Agreement”),
and (2) the Guaranty dated as of November 16, 1999 made by Frontier Holdings,
Inc., Frontier Refining & Marketing Inc., Frontier Refining Inc., Frontier El
Dorado Refining Company and Frontier Pipeline Inc. (the “Guarantors”) in favor
of the Lenders and the Agent, as amended by the First Amendment to Revolving
Credit Agreement and First Amendment to Guaranty dated September 20, 2000, the
Second Amendment to Revolving Credit Agreement and Second Amendment to Guaranty
and First Amendment to Clawback Agreement dated as of June 20, 2001, the Third
Amendment to Guaranty dated September 21, 2001, the Fourth Amendment to Guaranty
dated October 25, 2001 and the Third Amendment to Revolving Credit Agreement and
Notice Thereunder and Fifth Amendment to Guaranty dated January 30, 2002 (said
Guaranty, as so amended, herein called the “Guaranty”). Terms defined in the
Credit Agreement and not otherwise defined herein have the same respective
meanings when used herein, and the rules of interpretation set forth in Sections
1.2 and 1.3 of the Credit Agreement are incorporated herein by reference.

         1.  Subject to the terms and conditions of this letter amendment, the
Guarantors, the Lenders and the Agent hereby agree that the Guaranty is amended
are set forth below.

            (a)  The definition of “Liquidity Coverage Ratio” in Section 1 of
the Guaranty is amended by adding the parenthetical “(other than deposits or
other prepayments for purchases of foreign crude oil)” after the words “minus
prepaid expenses” in clause (a).

            (b)  The definition of “Tangible Net Worth” in Section 1 of the
Guaranty is amended by adding the parenthetical “(other than deposits or other
prepayments for purchases of foreign crude oil)” after the words “all prepaid
expenses” in clause (b).

            (c)   Section 8(j) of the Guaranty is amended in full to read as
follows:

          “(j)  Capital Expenditures. Such Guarantor will not make, or permit
any of its Subsidiaries to make, any expenditure for fixed or capital assets;
provided, however, that (i) FRMI and its Subsidiaries shall be permitted to make
such expenditures not exceeding (A) $25,000,000 in the aggregate in calendar
year 2001 and (B) $35,000,000 in the aggregate in each other calendar year and
(ii) in addition to the foregoing, FEDRC shall be permitted to make ‘Contingency
Earn-Up Payments’ to Equilon as required pursuant to the Acquisition Agreement.”

            (d)  Section 8(m) of the Guaranty is amended by amending in full the
proviso therein to read as follows:

  “provided, however, that (A) for purposes of determining compliance with
clause (i) above, EBITDA shall be calculated in accordance with the table set
forth below and (B) such Guarantor shall not be in default of this Section 8(m)
for the four-quarter period ending on September 30, 2002, even though the
consolidated EBITDA of FRMI and its Subsidiaries for such period is less than
$55,000,000, if and so long as (1) the consolidated EBITDA of FRMI and its
Subsidiaries for such period is at least $20,000,000 and (2) from and including
September 30, 2002 to but excluding the date after December 31, 2002 on which
FRMI delivers a financial report to the Lenders pursuant to Section 7(k)(ii) or
(iii) demonstrating compliance with this Section 8(m) (without reference to the
amount of Investible Cash maintained by FOC), FOC maintains Investible Cash in
an amount equal to or greater than the difference between $55,000,000 and the
consolidated EBITDA of FRMI and its Subsidiaries for the four-quarter period
ending on September 30, 2002.”

        2.  The Borrower, FOC and the Guarantors hereby represent and warrant
for the benefit of the Lenders and the Agent that (a) the representations and
warranties contained in the Credit Documents are correct in all material
respects on and as of the date of this letter amendment, before and after giving
effect to the same, as if made on and as of such date, and (b) no Default has
occurred and is continuing.

        3.  On and after the effective date of this letter amendment, each
reference in the Guaranty to “this Guaranty,” “hereunder,” “hereof,” “herein” or
words of like import referring to the Guaranty, and each reference in the other
Credit Documents to “the Guaranty,” “thereunder,” “thereof,” “therein” or words
of like import referring to the Guaranty, shall mean and be a reference to the
Guaranty as amended by this letter amendment. The Guaranty, as amended by this
letter amendment, is and shall continue to be in full force and effect and is
hereby ratified and confirmed in all respects.

        4.  By its execution below, FOC, as obligor under the Clawback
Agreement, hereby consents to this letter agreement and hereby confirms and
agrees that the Clawback Agreement is and shall continue to be in full force and
effect and is ratified and confirmed in all respects.

        5.  If you agree to the terms and conditions set forth herein, please
evidence your agreement by executing and returning 12 counterparts of this
letter amendment to the Agent. This letter amendment shall become effective as
of the date first set forth above when and if (a) the Borrower, FOC, the
Guarantors and the Majority Lenders execute and deliver to the Agent
counterparts of this letter amendment and (b) the Borrower pays to the Agent,
for the ratable account of each Lender approving this letter amendment on or
before the date first set forth above, an amendment fee in the amount equal to
0.05% of the aggregate amount of all such Lenders’ Commitments.

        6.  This letter amendment may be executed in any number of counterparts
and by any combination of the parties hereto in separate counterparts, each of
which counterparts shall be an original and all of which taken together shall
constitute one and the same letter amendment. Delivery of an executed
counterpart of a signature page to this letter amendment shall be effective as
delivery of an originally executed counterpart of this letter amendment.

        7.  THIS LETTER AMENDMENT SHALL BE GOVERNED BY, AND CONSTRUED AND
ENFORCED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF CALIFORNIA APPLICABLE TO
CONTRACTS MADE AND PERFORMED IN THE STATE OF CALIFORNIA.

  Very truly yours,

FRONTIER OIL CORPORATION

By:  /s/ Julie H. Edwards
      ——————————————————
      Julie H. Edwards
      Executive Vice President,
        Finance & Administration       FRONTIER HOLDINGS INC.

By: /s/ Julie H. Edwards
      ——————————————————
      Julie H. Edwards
      Executive Vice President,
        Finance & Administration
      FRONTIER REFINING & MARKETING INC.

By:  /s/ Leo J. Hoonakker
      ——————————————————
      Leo J. Hoonakker
      Treasurer       FRONTIER OIL AND REFINING COMPANY

By:  /s/ Leo J. Hoonakker
      ——————————————————
      Leo J. Hoonakker
      Treasurer       FRONTIER REFINING INC.

By:  /s/ Leo J. Hoonakker
      ——————————————————
      Leo J. Hoonakker
      Treasurer       FRONTIER EL DORADO REFINING COMPANY

By:  /s/ Leo J. Hoonakker
      ——————————————————
      Leo J. Hoonakker
      Treasurer       FRONTIER PIPELINE INC.

By:  /s/ Leo J. Hoonakker
      ——————————————————
      Leo J. Hoonakker
      Treasurer

Agreed as of the date first written above:

UNION BANK OF CALIFORNIA, N.A.,
as Administrative Agent and a Lender

By:  /s/ Randall L. Osterberg
      ——————————————————
      Randall L. Osterberg
      Senior Vice President

  BNP PARIBAS

By:  /s/ Douglas R. Liftman
      ——————————————————
      Douglas R. Liftman
      Managing Director

By:  /s/ Gabe Ellisor
      ——————————————————
      Gabe Ellisor
      Vice President

  TORONTO DOMINION (TEXAS), INC.

By:  /s/ Jano Nixon
      ——————————————————
      Jano Nixon
      Vice President

  BANK OF SCOTLAND

By:  /s/ Joseph Fratus
      ——————————————————
      Joseph Fratus
      First Vice President

  WELLS FARGO BANK, N.A.

By:  /s/ Art Krasny
      ——————————————————
      Art Krasny
      Relationship Manager

  THE BANK OF NOVA SCOTIA

By:  /s/ N. Bell
      ——————————————————
      N. Bell
      Senior Manager

  U.S. BANK NATIONAL ASSOCIATION

By:  /s/ Mark E. Thompson
      ——————————————————
      Mark E. Thompson
      Vice President

  FROST NATIONAL BANK

By:  /s/ Thomas H. Dungan
      ——————————————————
      Thomas H. Dungan
      Senior Vice President

  HIBERNIA NATIONAL BANK

By:  /s/ Nancy G. Moragas
      ——————————————————
      Nancy G. Moragas
      Vice President