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                                                                    Exhibit 10.6


                         RIO VISTA ENERGY PARTNERS L.P.
                              UNIT OPTION AGREEMENT

                            Void after July 10, 2006

Option No. ___                                      Date of Grant: July 10, 2003


     FOR VALUE RECEIVED and pursuant to the terms of that certain letter
agreement dated November 29, 2002, between Penn Octane Corporation, a Delaware
corporation ("Penn Octane") and Shore Capital LLC, the undersigned, RIO VISTA
ENERGY PARTNERS L.P., a limited partnership organized and existing under the
laws of the State of Delaware, hereby certifies that Shore Capital LLC is
entitled, subject to the terms set forth below, to purchase from the Partnership
after the completion of the distribution by Penn Octane to its stockholders of
all of the outstanding Common Units of the Partnership and before 5:00 P.M. New
York time, on July 10, 2006 (the "Expiration Date"), 97,415 Common Units of the
Partnership. The purchase price per Common Unit shall be $8.47 (the "Exercise
Price").

     The details of this Option Agreement (this "Agreement") are as follows:

     1.   DEFINITIONS.

          (A) "AFFILIATE" means, with respect to any specified person, any
person that directly or through one or more intermediaries controls or is
controlled by or is under common control with the specified person. As used in
this definition, the term "control" means the possession, directly or
indirectly, of the power to direct or cause the direction of the management and
policies of a person, whether through ownership of voting securities, by
contract or otherwise. For purposes of clarification, Penn Octane Corporation is
an Affiliate of the Partnership and the General Partner.

          (B) "CODE" means the Internal Revenue Code of 1986, as amended.

          (C) "GENERAL PARTNER" means Rio Vista GP LLC, a Delaware limited
liability company, and its successors or assigns.

          (D) "MANAGER" means a manager of the General Partner under the
Delaware Limited Liability Company Act.

          (E) "OFFICER" means any person designated by the General Partner as an
officer.

          (F) "OPTION" means an option to acquire Common Units granted pursuant
to this Agreement.

          (G) "OPTIONHOLDER" means the person to whom an Option to acquire
Common Units is granted pursuant to this Agreement.


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          (H) "PARTNERSHIP" means Rio Vista Energy Partners L.P., a Delaware
limited partnership, and its successors or assigns.

          (I) "PARTNERSHIP AGREEMENT" means the limited partnership agreement of
the Partnership, as amended.

          (J) "PURCHASE PRICE" means the amount equal to the product of the
Exercise Price and the number of Common Units to be purchased upon exercise of
this Option.

          (K) "COMMON UNIT" has the meaning set forth in the Partnership
Agreement or any equity interest into which a Common Unit is exchanged or
converted.

     2.   ADMINISTRATION.

          (A) ADMINISTRATION BY GENERAL PARTNER. The General Partner shall
administer this Agreement. Any interpretation of this Agreement by the General
Partner and any decision by the General Partner under this Agreement shall be
final and binding on all persons.

          (B) POWERS OF GENERAL PARTNER. The General Partner shall have the
power, subject to, and within the limitations of, the express provisions of this
Agreement:

               (i)  to construe and interpret this Agreement, and to establish,
amend and revoke rules and regulations for its administration; the General
Partner, in the exercise of this power, may correct any defect, omission or
inconsistency in this Agreement, in a manner and to the extent it shall deem
necessary or expedient to make this Agreement fully effective; and

               (ii) generally, to exercise such powers and to perform such acts
as the General Partner deems necessary or expedient to promote the best
interests of the Partnership which are not in conflict with the provisions of
this Agreement.

     3.   NUMBER OF COMMON UNITS, EXERCISE PRICE AND CAPITALIZATION ADJUSTMENTS.
The number of Common Units subject to this Option and the exercise price per
Common Unit may be adjusted from time to time for capitalization adjustments
described in this Section. If any change is made in the Common Units subject to
this Agreement, without the receipt of consideration by the Partnership (through
conversion, merger, consolidation, reorganization, recapitalization, unit
distribution, distribution in property other than cash, Common Units split,
liquidating distribution, combination of Common Units, exchange of Common Units,
change in structure or other transaction not involving the receipt of
consideration by the Partnership), the Common Units subject to this Agreement
will be appropriately adjusted in the class(es) and number of securities and
price per Common Unit. The General Partner, the determination of which shall be
final, binding and conclusive, shall make such adjustments. (The conversion of
any convertible securities of the Partnership shall not be treated as a
transaction "without receipt of consideration" by the Partnership.)

     4.   METHOD OF PAYMENT. Payment of the Purchase Price is due in full upon
exercise of all or any part of this Option. The Optionholder may elect to make
payment of the Purchase Price in cash or by check.

     5.   WHOLE COMMON UNITS. This Option may only be exercised for whole Common
Units.


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     6.   SECURITIES LAW COMPLIANCE. Notwithstanding anything to the contrary
contained herein, this Option may not be exercised unless the Common Units
issuable upon exercise of this Option are then registered under the Securities
Act or, if such Common Units are not then so registered, the Partnership has
determined that such exercise and issuance would be exempt from the registration
requirements of the Securities Act. The Partnership shall use its commercially
reasonable efforts to register such Common Units under the Securities Act.

     7.   TERM. The term of this Option commences on the date Penn Octane
completes the distribution to its stockholders of all the outstanding Common
Units of the Partnership and expires on the Expiration Date.

     8.   EXERCISE.

          (A) Subject to Section 8(c), the Optionholder may exercise this Option
during its term by delivering a Notice of Exercise (in a form designated by the
General Partner) together with the Purchase Price to the Secretary of the
General Partner, or to such other person as the General Partner may designate,
during regular business hours, together with such additional documents as the
General Partner may then require.

          (B) By exercising this Option, the Optionholder agrees that, as a
condition to any exercise of this Option, the Partnership or the General Partner
may require the Optionholder to enter an arrangement providing for the payment
by the Optionholder to the Partnership of any tax withholding obligation of the
Partnership, as reasonably determined by the Partnership, arising by reason of
(1) the exercise of this Option, or (2) the disposition of Common Units acquired
upon such exercise.

          (C) Notwithstanding anything herein to the contrary, during the term
of this Option, this Option may only be exercised during the first ten (10) days
of the first month of each fiscal quarter of the Partnership and during such
other periods as the General Partner may designate in its sole discretion for
purposes of minimizing the accounting costs to the Partnership resulting from
such exercise.

     9.   TRANSFERABILITY. This Option is not transferable, except to any
Affiliate of the Optionholder, and is exercisable only by the Optionholder or
such Affiliate. The transferability of the Common Units issued upon exercise of
this Option shall be subject to the conditions, restrictions and limitations set
forth in this Agreement, the Partnership Agreement and any other agreements the
Optionholder may have with the Partnership.

          (A) The provisions of this Section 9 may be waived with respect to any
transfer by the Partnership, upon duly authorized action of the General Partner.

          (B) Any sale or transfer, or purported sale or transfer, of the Common
Units by the Optionholder shall be null and void unless the terms, conditions,
and provisions of this Section 9 are observed and followed.


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     10.  WITHHOLDING OBLIGATIONS

          (A) At the time this Option is exercised, in whole or in part, or at
any time thereafter as requested by the Partnership, you hereby authorize
withholding from payroll and any other amounts payable to you, and otherwise
agree to make adequate provision for, any sums required to satisfy the federal,
state, local and foreign tax withholding obligations of the Partnership or an
Affiliate, if any, which arise in connection with this Option.

          (B) This Option is not exercisable unless the tax withholding
obligations of the Partnership and/or any Affiliate are satisfied. Accordingly,
you may not be able to exercise this Option when desired even though this Option
is vested, and the Partnership shall have no obligation to issue a certificate
for such Common Units or release such Common Units from any escrow provided for
herein.

     11.  TAX CONSEQUENCES AND CAPITAL ACCOUNT UPON EXERCISE OF OPTION. It is
intended that neither the Optionholder, the Partnership nor any of the partners
(owners) of the Partnership shall have any federal income tax consequences upon
grant of this Option, or, except as described herein, upon exercise of this
Option. Additionally, it is intended that the Optionholder shall not be treated
as a partner with respect to the Common Units subject to this Agreement until
such time as this Option is exercised. It is intended that, upon exercise of
this Option, the Optionholder shall include in income ("Exercise Income") in the
tax year of exercise an amount equal to the excess, if any, of the fair market
value of Common Units on the date of exercise over the sum of the amount the
Optionholder has paid or will pay to the Partnership ("Option Consideration")
upon the grant and exercise of the Option. It is intended that the Partnership
shall be entitled to a deduction equal to the amount of Exercise Income, which,
if the Partnership continues to be a partnership for federal income tax
purposes, shall be allocated solely to the partners of the Partnership other
than the Optionholder to whom the Exercise Income is attributable. Upon exercise
of this Option, Optionholder shall receive a positive capital account in the
Partnership (assuming the Partnership continues to be classified as a
partnership for federal income purposes) equal to the sum of the Exercise Income
to the Optionholder and the amount of Option Consideration paid by the
Optionholder. Should the federal income tax consequences differ from that
described in this section, the General Partner shall have the authority to
change the capital account or allocations of income and deduction to the
Optionholder as described herein in manner which in its discretion most closely
achieves the same economic effect of the consequences described in this Section.
Notwithstanding anything herein to the contrary, the General Partner shall have
the authority to change the tax reporting described above with respect to the
issuance or exercise of this Option or alter the capital accounts or allocations
of Partnership items as necessary to preserve or achieve the uniformity of
Common Units.

     12.  NOTICES. Any notices provided for in this Option or this Agreement
shall be given in writing and shall be deemed effectively given upon receipt or,
in the case of notices delivered by the Partnership to the Optionholder, five
(5) days after deposit in the United States mail, postage prepaid, addressed to
the Optionholder at the last address provided to the Partnership.


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     13.  THIS AGREEMENT; PARTNERSHIP AGREEMENT. This Option is subject to all
the provisions of this Agreement, the provisions of which are hereby made a part
of this Option, and is further subject to all interpretations, amendments, rules
and regulations which may from time to time be promulgated and adopted pursuant
to this Agreement, and the Common Units to be delivered are subject to the terms
of the Partnership Agreement. In the event of any conflict among Optionholder's
rights under the Common Units, this Agreement and the Partnership Agreement, the
terms of the Partnership Agreement shall control.

     14.  MISCELLANEOUS.

          (A) AVAILABILITY OF COMMON UNITS. During the term of this Option, the
Partnership shall keep available at all times the number of Common Units
required to satisfy the Option.

          (B) PARTNER RIGHTS. No Optionholder shall be deemed to be partner of
the Partnership, or to have any of the rights of a partner or rights of an
assignee from a partner with respect to, any Common Units subject to the Option
unless and until such Optionholder has satisfied all requirements for exercise
of the Option pursuant to its terms.

          (C) INVESTMENT ASSURANCES. Unless the Common Units issuable upon
exercise of this Option are then registered under the Securities Act, the
General Partner may require an Optionholder, as a condition of exercising or
acquiring Common Units under the Option, to give written assurances that he is
an "accredited investor," as defined in the rules and regulations under the
Securities Act. The General Partner may, upon advice of counsel to the General
Partner, place legends on Common Units certificates issued under the Option as
such counsel deems necessary or appropriate in order to comply with applicable
securities laws, including, but not limited to, legends restricting the transfer
of the Common Units.

          (D) PARTNERSHIP AGREEMENT. Before receiving Common Units, the
Optionholder shall take such action and execute such documents as the General
Partner may require to become a partner of the Partnership.

     IN WITNESS WHEREOF, this Option Agreement has been duly executed by the
parties hereto as of the Date of Grant.

                                           RIO VISTA ENERGY PARTNERS L.P.

                                           By: RIO VISTA GP LLC, general partner


                                               By:
                                                   -----------------------------
                                               Name:
                                                      --------------------------
                                               Title:
                                                      --------------------------


ACCEPTED AND AGREED:

SHORE CAPITAL LLC


By:
   -----------------------------------
      Richard Shore, Jr., President




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