March 17, 1993

To the Banks parties to the Revolving Credit and
Letter of Credit Agreement dated as of August 10,
1992 among Frontier Oil and Refining Company, said
Banks and Union Bank, as Agent for said Banks

Re: Waiver and Amendment

Gentlemen:

     We refer to the Revolving Credit and Letter of Credit Agreement dated as of
August 10, 1992 (the "Credit Agreement") among (1) Frontier Oil and Refining
Company (the "Borrower"), (2) each of Union Bank, Banque Paribas and Den norske
Bank AS (the "Banks") and (3) Union Bank, as agent for the Banks. Unless
otherwise defined herein, the terms defined in the Credit Agreement are used
herein as therein defined.

     FOC has requested that the Banks waive the requirements of Section 7.2(a)
of the FOC Guaranty as set forth below and amend Schedule 2 to the FOC Guaranty,
and the Borrower has requested that the Banks amend certain provisions of the
Credit Agreement concerning Inventory Audits.

     1.   The Banks hereby waive the requirements of Section 7.2(a) of the FOC
Guaranty during the period from the date hereof to July 31, 1993, but only to
the extent that such requirements are violated by Liens either now or hereafter
existing in favor of Petro Engineering and Construction, Inc., Asbestos Free
Insulation Contracting, Inc., Certified Welding, Heatec and certain union
workers, as disclosed in the letter dated February 25, 1993 from Jon D. Galvin
to Richard P. DeGrey, Jr. and the letter dated February 23, 1993 from James R.
Belcher, for Holland & Hart, to Mr. DeGrey, copies of which letters are attached
hereto as Exhibit A. Such waiver is conditioned upon the accuracy of the
information contained in the aforementioned letters, and any material adverse
change or material inaccuracy in such information shall cause such waiver to be
of no further force or effect.

     2.   Effective as of November 1, 1992, Schedule 2 to the FOC Guaranty is
hereby deleted and replaced by Schedule 2 hereto.
March 17, 1993
Page 2

     3.   The definition of "Inventory Audit" in Section 1.1 of the Credit
Agreement is hereby amended by deleting the word "monthly" therein.

     4.   Section 6.2 of the Credit Agreement is hereby amended by deleting
the word "monthly" therein and adding the words "every other month" after the
words "Inventory Audits."

     5.   FOC and the Borrower hereby represent and warrant for the benefit of
the Banks that (a) the representations and warranties contained in each Credit
Document are correct in all material respects on and as of the date of this
letter of waiver and amendment, before and after giving effect to the same, as
if made on and as of such date and (b) no event has occurred and is continuing
(except as waived in paragraph 1 above), or would result from the effectiveness
of this letter of waiver and amendment, that constitutes a Default or Event of
Default.

     6.   On and after the effective date of this letter of waiver and
amendment, (a) each reference in the FOC Guaranty to "this Guaranty,"
"hereunder," "hereof" or words of like import referring to the FOC Guaranty, (b)
each reference in the Credit Agreement to "this Agreement," "hereunder," hereof"
or words of like import referring to the Credit Agreement and (c) each reference
in the Credit Documents to "the FOC Guaranty," "the Credit Agreement,"
"thereunder," "thereof" or words of like import referring to the FOC Guaranty or
the Credit Agreement shall mean and be a reference to the FOC Guaranty or the
Credit Agreement, as applicable, as amended by this letter of waiver and
amendment. The FOC Guaranty and the Credit Agreement, both as amended by this
letter of waiver and amendment, and all of the other Credit Documents are and
shall continue to be in full force and effect and are hereby ratified and
confirmed in all respects.

     7.   The execution, delivery and effectiveness of this letter of waiver
and amendment shall not, except as expressly provided herein, operate as a
waiver of any right, power or remedy of any Bank or the Agent under any of the
Credit Documents or constitute a waiver of any provision of any of the Credit
documents .

     8.   This letter of waiver and 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 of waiver and
amendment.
March 17, 1993
Page 3

     9.   This letter of waiver and amendment shall become effective as of the
date first written above (except as otherwise provided in paragraph 2 above)
when and if counterparts hereof have been executed and delivered by FOC, the
Borrower, the Banks, Wainoco and the other Guarantors.

                              Very truly yours,

                              FRONTIER OIL CORPORATION


                              By: /s/  Jon D. Galvin
                              Vice President and Chief
                                Financial Officer

                              FRONTIER OIL AND REFINING COMPANY

                              By: /s/  Jon D. Galvin
                              Vice President and Chief
                                Financial Officer

Agreed as of the date first written above:

UNION BANK

By: /s/ Richard P. DeGrey, Jr.
Vice President

By: /s/ Philip B. Flynn
Senior Vice President