Document:

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                                                                   EXHIBIT 4.3.5

                                     WARRANT

THE WARRANT EVIDENCED OR CONSTITUTED HEREBY, AND ALL SHARES OF COMMON STOCK
ISSUABLE HEREUNDER, HAVE BEEN AND WILL BE ISSUED WITHOUT REGISTRATION UNDER THE
SECURITIES ACT OF 1933, AS AMENDED ("THE ACT") AND MAY NOT BE SOLD, OFFERED FOR
SALE, TRANSFERRED, PLEDGED OR HYPOTHECATED WITHOUT REGISTRATION UNDER THE ACT
UNLESS EITHER (i) THE COMPANY HAS RECEIVED AN OPINION OF COUNSEL, IN FORM AND
SUBSTANCE REASONABLY SATISFACTORY TO THE COMPANY, TO THE EFFECT THAT
REGISTRATION IS NOT REQUIRED IN CONNECTION WITH SUCH DISPOSITION OR (ii) THE
SALE OF SUCH SECURITIES IS MADE PURSUANT TO SECURITIES AND EXCHANGE COMMISSION
RULE 144.

             WARRANT TO PURCHASE COMMON STOCK OF PROBEX CORPORATION

                             (Subject to Adjustment)

NO. O

         THIS CERTIFIES THAT, for value received, HSB Engineering Finance
Corporation, or its permitted registered assigns ("HOLDER"), is entitled,
subject to the terms and conditions of this Warrant, at any time or from time to
time after June 30, 1998 (the "EFFECTIVE DATE"), and before 5:00 p.m. Central
Time on June 30, 2008 (the "EXPIRATION DATE"), to purchase from Probex
Corporation, a Colorado corporation (the "COMPANY"), _________ shares of Common
Stock of the Company at a price per share of $0.01 (the "PURCHASE PRICE"). Both
the number of shares of Common Stock purchasable upon exercise of this Warrant
and the Purchase Price are subject to adjustment and change as provided herein.
This Warrant is issued pursuant to the Convertible Loan, Warrant and Security
Agreement, dated June 30, 1998 (the "LOAN AGREEMENT"), between the Company and
Holder.

1. CERTAIN DEFINITIONS. As used in this Warrant the following terms shall have
the following respective meanings:

         "Actively Traded" shall mean attainment of an average of 37,500 or more
shares per business day arm's length trading volume of Common Stock over a
period of twenty (20) trading days as reported by the over-the-counter or other
established quotation agencies.

         "Fair Market Value" of a share of Common Stock as of a particular date
shall mean:

                  (a) If traded on a securities exchange or the Nasdaq National
Market, the Fair Market Value shall be deemed to be the average of the closing
prices of the Common Stock of the Company on such exchange or market over the 5
business days ending immediately prior to the applicable date of valuation;

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                  (b) If Actively Traded over-the-counter, the Fair Market Value
shall be deemed to be the average of the closing bid prices over the 30-day
period ending immediately prior to the applicable date of valuation; and

                  (c) If not Actively Traded over-the-counter or traded on a
securities exchange or the Nasdaq National Market, the Fair Market Value shall
be the value thereof, as agreed upon by the Company and the Holder; provided,
however, that if the Company and the Holder cannot agree on such value, such
value shall be determined by an independent valuation firm experienced in
valuing businesses such as the Company and jointly selected in good faith by the
Company and the Holder. Fees and expenses of the valuation firm shall be paid
for by the Company.

         "Fully-Diluted" shall mean after giving effect to the exercise or
conversion, as applicable, of all outstanding options, warrants, and other
securities convertible or exchangeable for Common Stock.

         "HSR Act" shall mean the Hart-Scott-Rodino Antitrust Improvements Act
of 1976, as amended.

         "Public Offering" shall mean after the date hereof any firm commitment
underwritten public offering of the Company's Common Stock pursuant to a
registration statement filed with the Securities and Exchange Commission.

         "Registered Holder" shall mean any Holder in whose name this Warrant is
registered upon the books and records maintained by the Company.

         "Warrant" as used herein, shall include this Warrant and any warrant
delivered in substitution or exchange therefor as provided herein.

         "Warrant Shares" shall mean the shares of Common Stock to be issuable
upon exercise of this Warrant (or any shares of stock or other securities at the
time issuable upon exercise of this Warrant).

         "Common Stock" shall mean the Common Stock of the Company and any other
securities at any time receivable or issuable upon exercise of this Warrant.

2. EXERCISE OF WARRANT

         2.1. Payment. Subject to compliance with the terms and conditions of
this Warrant and applicable securities laws, this Warrant may be exercised, in
whole or in part at any time or from time to time, on or before the Expiration
Date by the delivery (including, without limitation, delivery by facsimile) of
the form of Notice of Exercise attached hereto as Exhibit 1 (the "NOTICE OF
EXERCISE"), duly executed by the Holder, at the principal office of the Company,
and as soon as practicable after such date, surrendering

                  (a) this Warrant at the principal office of the Company, and

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                  (b) payment, (i) in cash (by check) or by wire transfer, (ii)
by cancellation by the Holder of indebtedness of the Company to the Holder; or
(iii) by a combination of (i) and (ii), of an amount equal to the product
obtained by multiplying the number of shares of Common Stock being purchased
upon such exercise by the then effective Purchase Price (the "EXERCISE AMOUNT"),
except that if Holder is subject to HSR Act Restrictions (as defined in Section
2.4 below), the Exercise Amount shall be paid to the Company within five (5)
business days of the termination of all HSR Act Restrictions.

         2.2. Net Issue Exercise. In lieu of the payment methods set forth in
Section 2.1(b) above, the Holder may elect to exchange all or some of the
Warrant for shares of Common Stock equal to the value of the amount of the
Warrant being exchanged on the date of exchange. If Holder elects to exchange
this Warrant as provided in this Section 2.2, Holder shall tender to the Company
the Warrant for the amount being exchanged, along with written notice of
Holder's election to exchange some or all of the Warrant, and the Company shall
issue to Holder the number of shares of the Common Stock computed using the
following formula:

                  X = Y (A-B)
                      -------
                         A

                  Where X = the number of shares of Common Stock to be issued to
                  Holder.

                  Y = the number of shares of Common Stock purchasable under the
                  amount of the Warrant being exchanged (as adjusted to the date
                  of such calculation).

                  A = the Fair Market Value of one share of the Company's Common
                  Stock.

                  B = Purchase Price (as adjusted to the date of such
                  calculation).

                  All references herein to an "exercise" of the Warrant shall
include an exchange pursuant to this Section 2.2. Upon receipt of a written
notice of the Company's intention to raise capital by selling shares of Common
Stock in a Public Offering (the "PUBLIC OFFERING NOTICE"), which notice shall be
delivered to Holder at least forty-five (45) but not more than ninety (90) days
before the anticipated date of the filing with the Securities and Exchange
Commission of the registration statement associated with the Public Offering,
the Holder shall promptly notify the Company whether or not the Holder will
exercise this Warrant pursuant to this Section 2.2 prior to consummation of the
Public Offering. Notwithstanding whether or not a Public Offering Notice has
been delivered to Holder or any other provision of this Warrant to the contrary,
if Holder decides to exercise this Warrant while a registration statement is on
file with the Securities and Exchange Commission (the "SEC") in connection with
the Public Offering, this Warrant shall be deemed exercised on the consummation
of the Public Offering and the Fair Market Value of a share of Common Stock will
be the price at which one share of Common Stock was sold to the public in the
Public Offering. If Holder has elected to exercise this Warrant pursuant to this
Section 2.2 while a registration statement is on file with the Securities and
Exchange Commission in connection with a Public Offering and the Public Offering
is not consummated, then Holder's exercise of this Warrant shall not be
effective unless Holder confirms in writing Holder's intention to go forward
with the exercise of this Warrant.

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         2.3. "Easy Sale" Exercise. In lieu of the payment methods set forth in
Section 2.1(b) above, when permitted by law and applicable regulations
(including Nasdaq and NASD rules), the Holder may pay the Purchase Price through
a "same day sale" commitment from the Holder (and if applicable a broker-dealer
that is a member of the National Association of Securities Dealers (a "NASD
DEALER")), whereby the Holder irrevocably elects to exercise this Warrant and to
sell a portion of the Shares so purchased to pay for the Purchase Price and the
Holder (or, if applicable, the NASD Dealer) commits upon sale (or, in the case
of the NASD Dealer, upon receipt) of such Shares to forward the Purchase Price
directly to the Company.

         2.4. Stock Certificates; Fractional Shares. As soon as practicable on
or after such date, the Company shall issue and deliver to the person or persons
entitled to receive the same a certificate or certificates for the number of
whole shares of Common Stock issuable upon such exercise, together with cash in
lieu of any fraction of a share equal to such fraction of the current Fair
Market Value of one whole share of Common Stock as of the date of exercise of
this Warrant. No fractional shares or scrip representing fractional shares shall
be issued upon an exercise of this Warrant.

         2.5. HSR Act. The Company hereby acknowledges that exercise of this
Warrant by Holder may subject the Company and/or the Holder to the filing
requirements of the HSR Act and that Holder may be prevented from exercising
this Warrant until the expiration or early termination of all waiting periods
imposed by the HSR Act ("HSR ACT RESTRICTIONS"). If on or before the Expiration
Date Holder has sent the Notice of Exercise to Company and Holder has not been
able to complete the exercise of this Warrant prior to the Expiration Date
because of HSR Act Restrictions, the Holder shall be entitled to complete the
process of exercising this Warrant in accordance with the procedures contained
herein notwithstanding the fact that completion of the exercise of this Warrant
would take place after the Expiration Date or the completion of the Public
Offering.

         2.6. Partial Exercise; Effective Date of Exercise. In case of any
partial exercise of this Warrant, the Company shall cancel this Warrant upon
surrender hereof and shall execute and deliver a new Warrant of like tenor and
date for the balance of the shares of Common Stock purchasable hereunder. This
Warrant shall be deemed to have been exercised immediately prior to the close of
business on the date of its surrender for exercise as provided above. However,
if Holder is subject to HSR Act filing requirements this Warrant shall be deemed
to have been exercised on the date immediately following the date of the
expiration of all HSR Act Restrictions. The person entitled to receive the
shares of Common Stock issuable upon exercise of this Warrant shall be treated
for all purposes as the holder of record of such shares as of the close of
business on the date the Holder is deemed to have exercised this Warrant.

3. VALID ISSUANCE: TAXES. All shares of Common Stock issued upon the exercise of
this Warrant shall be validly issued, fully paid and non-assessable, and the
Company shall pay all taxes and other governmental charges that may be imposed
in respect of the issue or delivery thereof. The Company shall not be required
to pay any tax or other charge imposed in connection with any transfer involved
in the issuance of any certificate for shares of Common Stock in any name other
than that of the Registered Holder of this Warrant, and in such case the Company
shall not be required to issue or deliver any stock certificate or security
until such tax

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or other charge has been paid, or it has been established to the Company's
reasonable satisfaction that no tax or other charge is due.

4. ADJUSTMENT OF PURCHASE PRICE AND NUMBER OF SHARES. The number of shares of
Common Stock issuable upon exercise of this Warrant (or any shares of stock or
other securities or property receivable or issuable upon exercise of this
Warrant) and the Purchase Price are subject to adjustment upon occurrence of the
following events:

         4.1. Adjustment for Stock Splits, Stock Subdivisions or Combinations of
Shares. The Purchase Price of this Warrant shall be proportionally decreased and
the number of Warrant Shares shall be proportionally increased to reflect any
stock split or subdivision of the Company's Common Stock. The Purchase Price of
this Warrant shall be proportionally increased and the number of Warrant Shares
shall be proportionally decreased to reflect any combination of the Company's
Common Stock.

         4.2. Adjustment for Dividends or Distributions of Stock or Other
Securities or Property. In case the Company shall make or issue, or shall fix a
record date for the determination of eligible holders entitled to receive, a
dividend or other distribution with respect to the Common Stock (or any shares
of stock or other securities at the time issuable upon exercise of the Warrant)
payable in (a) securities of the Company or (b) assets (excluding cash dividends
paid or payable solely out of funds legally available therefor), then, in each
such case, the Holder of this Warrant on exercise hereof at any time after the
consummation, effective date or record date of such dividend or other
distribution, shall receive, in addition to the shares of Common Stock (or such
other stock or securities) issuable on such exercise prior to such date, and
without the payment of additional consideration therefor, the securities or such
other assets of the Company to which such Holder would have been entitled upon
such date if such Holder had exercised this Warrant on the date hereof and had
thereafter, during the period from the date hereof to and including the date of
such exercise, retained such shares and/or all other additional stock available
by it as aforesaid during such period giving effect to all adjustments called
for by this Section 4.

         4.3. Reclassification. If the Company, by reclassification of
securities or otherwise, shall change any of the securities as to which purchase
rights under this Warrant exist into the same or a different number of
securities of any other class or classes, this Warrant shall thereafter
represent the right to acquire such number and kind of securities as would have
been issuable as the result of such change with respect to the securities that
were subject to the purchase rights under this Warrant immediately prior to such
reclassification or other change and the Purchase Price therefore shall be
appropriately adjusted, all subject to further adjustment as provided in this
Section 4. No adjustment shall be made pursuant to this Section 4.3 upon any
conversion or redemption of the Common Stock which is the subject of Section
4.5.

         4.4. Adjustment for Capital Reorganization, Merger or Consolidation. In
case of any capital reorganization of the capital stock of the Company (other
than a combination, reclassification, exchange or subdivision of shares
otherwise provided for herein), or any merger or consolidation of the Company
with or into another corporation, or the sale of all or substantially all the
assets of the Company then, and in each such case, as a part of such
reorganization, merger, consolidation, sale or transfer, lawful provision shall
be made so that the

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Holder of this Warrant shall thereafter be entitled to receive upon exercise of
this Warrant, during the period specified herein and upon payment of the
Purchase Price then in effect, the number of shares of stock or other securities
or property of the successor corporation resulting from such reorganization,
merger, consolidation, sale or transfer that a holder of the shares deliverable
upon exercise of this Warrant would have been entitled to receive in such
reorganization, consolidation, merger, sale or transfer if this Warrant had been
exercised immediately before such reorganization, merger, consolidation, sale or
transfer, all subject to further adjustment as provided in this Section 4. The
foregoing provisions of this Section 4.4 shall similarly apply to successive
reorganizations, consolidations, mergers, sales and transfers and to the stock
or securities of any other corporation that are at the time receivable upon the
exercise of this Warrant. If the per-share consideration payable to the Holder
hereof for shares in connection with any such transaction is in a form other
than cash or marketable securities, then the value of such consideration shall
be determined in good faith by the Company's Board of Directors. In all events,
appropriate adjustment (as determined in good faith by the Company's Board of
Directors) shall be made in the application of the provisions of this Warrant
with respect to the rights and interests of the Holder after the transaction, to
the end that the provisions of this Warrant shall be applicable after that
event, as near as reasonably may be, in relation to any shares or other property
deliverable after that event upon exercise of this Warrant.

         4.5. Conversion of Common Stock. In case all or any portion of the
authorized and outstanding shares of Common Stock of the Company are redeemed or
converted or reclassified into other securities or property pursuant to the
Company's Articles of Incorporation or otherwise, or the Common Stock otherwise
ceases to exist, then, in such case, the Holder of this Warrant, upon exercise
hereof at any time after the date on which the Common Stock is so redeemed or
converted, reclassified or ceases to exist (the "TERMINATION DATE"), shall
receive, in lieu of the number of shares of Common Stock that would have been
issuable upon such exercise immediately prior to the Termination Date, the
securities or property that would have been received if this Warrant had been
exercised in full and the Common Stock received thereupon had been
simultaneously converted immediately prior to the Termination Date, all subject
to further adjustment as provided in this Warrant. Additionally, the Purchase
Price shall be immediately adjusted to equal the quotient obtained by dividing
(x) the aggregate Purchase Price of the maximum number of shares of Common Stock
for which this Warrant was exercisable immediately prior to the Termination Date
by (y) the number of shares of Common Stock of the Company for which this
Warrant is exercisable immediately after the Termination Date, all subject to
further adjustment as provided herein.

         4.6. Adjustment for Issuance of Additional Shares of Common Stock.
Until consummation of a Public Offering, upon issuance by the Company of (1)
Common Stock, (2) any right, option or warrant to acquire Common Stock, (3) any
other stock convertible into Common Stock, or (4) any obligation or any share of
stock convertible into or exchangeable for Common Stock (including the
Additional Conversion Shares but excluding any Conversion Shares issued pursuant
to Section 5.7.6 of the Loan Agreement and any Warrant Shares adjusted pursuant
to this Section) (in the case of (2), (3) and (4), collectively, "Common Stock
Equivalents"), the number of Warrant Shares shall be adjusted as follows:

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                  (a) if (i) Common Stock is issued and the consideration
received is (x) other than cash (including personal checks) or other immediately
available funds, or (y) cash (including personal checks) or other immediately
available funds in an amount less than $1.00 per share, or (ii) Common Stock
Equivalents are issued and the exercise, conversion or exchange price per share
of Common Stock is less than $1.00 per share of Common Stock, then the number of
Warrant Shares shall be increased by the following amount:

<TABLE>
<S>                        <C>
   Number of Shares of        Number of Warrant Shares Outstanding Prior to Issuance
  Common Stock Issued or   x
Issuable Upon Exercise of    -------------------------------------------------------
 Common Stock Equivalents                                -
                                   Number of Shares of Common Stock Outstanding
                                         (Fully-Diluted) Prior to Issuance
</TABLE>

                  (b) if (i) Common Stock is issued and the consideration
received is (x) cash (including personal checks) or other immediately available
funds in an amount equal to or greater than $1.00 per share but less than $1.50
per share, or (ii) Common Stock Equivalents are issued and the exercise,
conversion or exchange price per share of Common Stock is equal to or greater
than $1.00 per share but less than $1.50 per share of Common Stock, then the
number of Warrant Shares shall be increased by the following amount:

<TABLE>
<S>                        <C>
    Number of Shares of       Number of Warrant Shares Outstanding Prior to Issuance
  Common Stock Issued or   x                                                           x   0.50
Issuable Upon Exercise of    -------------------------------------------------------       0
 Common Stock Equivalents          Number of Shares of Common Stock Outstanding
                                         (Fully-Diluted) Prior to Issuance
</TABLE>

5. CERTIFICATE AS TO ADJUSTMENTS. In each case of any adjustment in the Purchase
Price, or number or type of shares issuable upon exercise of this Warrant, the
Chief Financial Officer or Controller of the Company shall compute such
adjustment in accordance with the terms of this Warrant and prepare a
certificate setting forth such adjustment and showing in detail the facts upon
which such adjustment is based, including a statement of the adjusted Purchase
Price. The Company shall promptly send (by facsimile and by either first class
mail, postage prepaid or overnight delivery) a copy of each such certificate to
the Holder.

6. LOSS OR MUTILATION. Upon receipt of evidence reasonably satisfactory to the
Company of the ownership of and the loss, theft, destruction or mutilation of
this Warrant, and of indemnity reasonably satisfactory to it, and (in the case
of mutilation) upon surrender and cancellation of this Warrant, the Company will
execute and deliver in lieu thereof a new Warrant of like tenor as the lost,
stolen, destroyed or mutilated Warrant.

7. RESERVATION OF COMMON STOCK. The Company hereby covenants that at all times
there shall be reserved for issuance and delivery upon exercise of this Warrant
such number of shares of Common Stock or other shares of capital stock of the
Company as are from time to time issuable upon exercise of this Warrant and,
from time to time, will take all steps necessary to amend its Articles of
Incorporation to provide sufficient reserves of shares of Common Stock issuable
upon exercise of this Warrant (and shares of its Common Stock for issuance on
conversion of such Common Stock). All such shares shall be duly authorized, and
when issued upon such exercise, shall be validly issued, fully paid and
non-assessable, free and

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clear of all liens, security interests, charges and other encumbrances or
restrictions on sale and free and clear of all preemptive rights, except
encumbrances or restrictions arising under federal or state securities laws.
Issuance of this Warrant shall constitute full authority to the Company's
officers who are charged with the duty of executing stock certificates to
execute and issue the necessary certificates for shares of Common Stock and
Common Stock upon the exercise of this Warrant.

8. TRANSFER AND EXCHANGE. Subject to the terms and conditions of this Warrant
and compliance with all applicable securities laws, this Warrant and all rights
hereunder may be transferred in whole or in part, on the books of the Company
maintained for such purpose at the principal office of the Company referred to
above, to any Registered Holder parent, subsidiary or affiliate, by the
Registered Holder hereof in person, or by duly authorized attorney, upon
surrender of this Warrant properly endorsed and upon payment of any necessary
transfer tax or other governmental charge imposed upon such transfer. Upon any
permitted partial transfer, the Company will issue and deliver to the Registered
Holder a new Warrant or Warrants with respect to the shares of Common Stock not
so transferred. Each taker and holder of this Warrant, by taking or holding the
same, consents and agrees that when this Warrant shall have been so endorsed,
the person in possession of this Warrant may be treated by the Company, and all
other persons dealing with this Warrant, as the absolute owner hereof for any
purpose and as the person entitled to exercise the rights represented hereby,
any notice to the contrary notwithstanding; provided, however that until a
transfer of this Warrant is duly registered on the books of the Company, the
Company may treat the Registered Holder hereof as the owner for all purposes.

9. RESTRICTIONS ON TRANSFER. The Holder, by acceptance hereof, agrees that,
absent an effective registration statement filed with the SEC under the
Securities Act of 1933, as amended (the "1933 ACT"), covering the disposition or
sale of this Warrant or the Common Stock issued or issuable upon exercise hereof
or the Common Stock issuable upon conversion thereof, as the case may be, and
registration or qualification under applicable state securities laws, such
Holder will not sell, transfer, pledge, or hypothecate any or all such Warrants
or Common Stock, as the case may be, unless either (i) the Company has received
an opinion of counsel, in form and substance reasonably satisfactory to the
Company, to the effect that such registration is not required in connection with
such disposition or (ii) the sale of such securities is made pursuant to SEC
Rule 144.

10. COMPLIANCE WITH SECURITIES LAWS. By acceptance of this Warrant, the holder
hereby represents, warrants and covenants that any shares of stock purchased
upon exercise of this Warrant or acquired upon conversion thereof shall be
acquired for investment only and not with a view to, or for sale in connection
with, any distribution thereof; that the Holder has had such opportunity as such
Holder has deemed adequate to obtain from representatives of the Company such
information as is necessary to permit the Holder to evaluate the merits and
risks of its investment in the company; that the Holder is able to bear the
economic risk of holding such shares as may be acquired pursuant to the exercise
of this Warrant for an indefinite period; that the Holder understands that the
shares of stock acquired pursuant to the exercise of this Warrant or acquired
upon conversion thereof will not be registered under the 1933 Act (unless
otherwise required pursuant to exercise by the Holder of the registration
rights,

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if any, previously granted to the registered Holder) and will be "restricted
securities" within the meaning of Rule 144 under the 1933 Act and that the
exemption from registration under Rule 144 will not be available for at least
one year from the date of exercise of this Warrant, subject to any special
treatment by the SEC for exercise of this Warrant pursuant to Section 2.2, and
even then will not be available unless a public market then exists for the
stock, adequate information concerning the Company is then available to the
public, and other terms and conditions of Rule 144 are complied with; and that
all stock certificates representing shares of stock issued to the Holder upon
exercise of this Warrant or upon conversion of such shares may have affixed
thereto a legend substantially in the following form:

         THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE
         SECURITIES ACT OF 1933, AS AMENDED (THE "ACT"), OR UNDER THE SECURITIES
         LAWS OF ANY STATE. THESE SECURITIES ARE SUBJECT TO RESTRICTIONS ON
         TRANSFERABILITY AND RESALE AND MAY NOT BE TRANSFERRED OR RESOLD EXCEPT
         AS PERMITTED UNDER THE ACT AND THE APPLICABLE STATE SECURITIES LAWS,
         PURSUANT TO REGISTRATION OR EXEMPTION THEREFROM. INVESTORS SHOULD BE
         AWARE THAT THEY MAY BE REQUIRED TO BEAR THE FINANCIAL RISKS OF THIS
         INVESTMENT FOR AN INDEFINITE PERIOD OF TIME. THE ISSUER OF THESE
         SECURITIES MAY REQUIRE AN OPINION OF COUNSEL IN FORM AND SUBSTANCE
         SATISFACTORY TO THE ISSUER TO THE EFFECT THAT ANY PROPOSED TRANSFER OR
         RESALE IS IN COMPLIANCE WITH THE ACT AND ANY APPLICABLE STATE
         SECURITIES LAWS.

11. NO RIGHTS OR LIABILITIES AS STOCKHOLDERS. This Warrant shall not entitle the
Holder to any voting rights or other rights as a stockholder of the Company. In
the absence of affirmative action by such Holder to purchase Common Stock by
exercise of this Warrant, no provisions of this Warrant, and no enumeration
herein of the rights or privileges of the Holder hereof shall cause such Holder
hereof to be a stockholder of the Company for any purpose.

12. REGISTRATION RIGHTS. All shares of Common Stock issuable upon exercise of
this Warrant shall be "REGISTRABLE SECURITIES" or such other definition of
securities entitled to registration rights pursuant to the Investor Rights
Agreement, dated as of the date hereof, between the Company and Holder, and are
entitled, subject to the terms and conditions of that agreement, to all
registration rights granted to holders of Registrable Securities thereunder.

13. NOTICES. All notices and other communications from the Company to the Holder
shall be given in accordance with the Loan Agreement.

14. HEADINGS. The headings in this Warrant are for purposes of convenience in
reference only, and shall not be deemed to constitute a part hereof.

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15. LAW GOVERNING. This Warrant shall be construed and enforced in accordance
with, and governed by, the laws of the State of Connecticut.

16. NO IMPAIRMENT. The Company will not, by amendment of its Articles of
Incorporation or bylaws, or through reorganization, consolidation, merger,
dissolution, issue or sale of securities, sale of assets or any other voluntary
action, avoid or seek to avoid the observance or performance of any of the terms
of this Warrant, but will at all times in good faith assist in the carrying out
of all such terms and in the taking of all such action as may be necessary or
appropriate in order to protect the rights of the Registered Holder of this
Warrant against impairment. Without limiting the generality of the foregoing,
the Company (a) will not increase the par value of any shares of stock issuable
upon the exercise of this Warrant above the amount payable therefor upon such
exercise, and (b) will take all such action as may be necessary or appropriate
in order that the Company may validly and legally issue fully paid and
non-assessable shares of Common Stock upon exercise of this Warrant.

17. NOTICES OF RECORD DATE. In case:

         17.1. the Company shall take a record of the holders of its Common
Stock (or other stock or securities at the time receivable upon the exercise of
this Warrant), for the purpose of entitling them to receive any dividend or
other distribution, or any right to subscribe for or purchase any shares of
stock of any class or any other securities or to receive any other right; or

         17.2. of any consolidation or merger of the Company with or into
another corporation, any capital reorganization of the Company, any
reclassification of the Capital Stock of the Company, or any conveyance of all
or substantially all of the assets of the Company to another corporation in
which holders of the Company's stock are to receive stock, securities or
property of another corporation; or

         17.3. of any voluntary dissolution, liquidation or winding-up of the
Company; or

         17.4. of any redemption or conversion of all outstanding Common Stock;

then, and in each such case, the Company will mail or cause to be mailed to the
Registered Holder of this Warrant a notice specifying, as the case may be, (i)
the date on which a record is to be taken for the purpose of such dividend,
distribution or right, or (ii) the date on which such reorganization,
reclassification, consolidation, merger, conveyance, dissolution, liquidation,
winding-up, redemption or conversion is to take place, and the time, if any is
to be fixed, as of which the holders of record of Common Stock or (such stock or
securities as at the time are receivable upon the exercise of this Warrant),
shall be entitled to exchange their shares of Common Stock (or such other stock
or securities), for securities or other property deliverable upon such
reorganization, reclassification, consolidation, merger, conveyance,
dissolution, liquidation or winding-up. Such notice shall be delivered at least
thirty (30) days prior to the date therein specified.

18. SEVERABILITY. If any term, provision, covenant or restriction of this
Warrant is held by a court of competent jurisdiction to be invalid, void or
unenforceable, the remainder of the

                                      -10-

WARRANT

<PAGE>   11
terms, provisions, covenants and restrictions of this Agreement shall remain in
full force and effect and shall in no way be affected, impaired or invalidated.

19. COUNTERPARTS. For the convenience of the parties, any number of counterparts
of this Warrant may be executed by the parties hereto and each such executed
counterpart shall be, and shall be deemed to be, an original instrument.

20. NO INCONSISTENT AGREEMENTS. The Company will not on or after the date of
this Warrant enter into any agreement with respect to its securities which is
inconsistent with the rights granted to the Holders of this Warrant or otherwise
conflicts with the provisions hereof. The rights granted to the Holders
hereunder do not in any way conflict with and are not inconsistent with the
rights granted to holders of the Company's securities under any other
agreements, except rights that have been waived.

21. SATURDAYS, SUNDAYS AND HOLIDAYS. If the Expiration Date falls on a Saturday,
Sunday or legal holiday, the Expiration Date shall automatically be extended
until 5:00 p.m. the next business day.

22. BINDING EFFECT. Pursuant to Section 14.17 of the Loan Agreement, this
Warrant shall not be binding upon either party until the Company successfully
conducts an Acceptance Test on crankcase used oil, as defined in and pursuant to
the Loan Agreement.

                            [SIGNATURES ON NEXT PAGE]

                                      -11-

WARRANT

<PAGE>   12

         IN WITNESS WHEREOF, the parties hereto have executed this Warrant as of
the Effective Date.

HSB ENGINEERING FINANCE CORPORATION,
A DELAWARE CORPORATION

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

PROBEX CORPORATION,
A COLORADO CORPORATION

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

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

                                      -12-

WARRANT

<PAGE>   13

                                    EXHIBIT 1

                               NOTICE OF EXERCISE

                    (To be executed upon exercise of Warrant)

PROBEX CORPORATION                                               WARRANT NO. ___

The undersigned hereby irrevocably elects to exercise the right of purchase
represented by the within Warrant Certificate for, and to purchase thereunder,
the securities of Probex Corporation, as provided for therein, and (check the
applicable box):

o        Tenders herewith payment of the exercise price in full in the form of
         cash or a certified or official bank check in same-day funds in the
         amount of $____________ for _________ such securities.

o        Elects the Net Issue Exercise option pursuant to Section 2.2 of the
         Warrant, and accordingly requests delivery of a net of ______________
         of such securities, according to the following calculation:

                  X = Y (A-B)         (       ) =  (____) [(_____) - (_____)]
                      -------                     ---------------------------
                         A                                   (_____)

                  Where X = the number of shares of Common Stock to be issued to
                  Holder.

                  Y = the number of shares of Common Stock purchasable under the
                  amount of the Warrant being exchanged (as adjusted to the date
                  of such calculation).

                  A = the Fair Market Value of one share of the Company's Common
                  Stock.

                  B = Purchase Price (as adjusted to the date of such
                  calculation).

o        Elects the Easy Sale Exercise option pursuant to Section 2.3 of the
         Warrant, and accordingly requests delivery of a net of ______________
         of such securities.

Please issue a certificate or certificates for such securities in the name of,
and pay any cash for any fractional share to (please print name, address and
social security number):

Name:
           ----------------------------------------
Address:
           ----------------------------------------
Signature:
           ----------------------------------------

Note: The above signature should correspond exactly with the name on the first
page of this Warrant Certificate or with the name of the assignee appearing in
the assignment form below.

If said number of shares shall not be all the shares purchasable under the
within Warrant Certificate, a new Warrant Certificate is to be issued in the
name of said undersigned for the balance remaining of the shares purchasable
thereunder rounded up to the next higher whole number of shares.

<PAGE>   14

                                    EXHIBIT 2

                                   ASSIGNMENT

(To be executed only upon assignment of Warrant                  WARRANT NO. ___
Certificate)

For value received, hereby sells, assigns and transfers unto
________________________ the within Warrant Certificate, together with all
right, title and interest therein, and does hereby irrevocably constitute and
appoint ____________________________ attorney, to transfer said Warrant
Certificate on the books of the within-named Company with respect to the number
of Warrants set forth below, with full power of substitution in the premises:

<TABLE>
<CAPTION>
====================================================================================================================
<S>                                                 <C>                                  <C>
        NAME(S) OF ASSIGNEE(S)                         ADDRESS                            # OF WARRANTS
--------------------------------------------------------------------------------------------------------------------

--------------------------------------------------------------------------------------------------------------------

--------------------------------------------------------------------------------------------------------------------

--------------------------------------------------------------------------------------------------------------------

--------------------------------------------------------------------------------------------------------------------

====================================================================================================================
</TABLE>

And if said number of Warrants shall not be all the Warrants represented by the
Warrant Certificate, a new Warrant Certificate is to be issued in the name of
said undersigned for the balance remaining of the Warrants registered by said
Warrant Certificate.

Dated:                                                              19
               ---------------------------------------------------------

Signature:
               ---------------------------------------------------------

Notice: The signature to the foregoing Assignment must correspond to the name as
written upon the face of this security in every particular, without alteration
or any change whatsoever; signature(s) must be guaranteed by an eligible
guarantor institution (banks, stock brokers, savings and loan associations and
credit unions with membership in an approved signature guarantee medallion
program) pursuant to Securities and Exchange Commission Rule 17Ad-15.<PAGE>   1
                                                                  EXHIBIT 4.3.12

THIS WARRANT AND ANY SECURITIES ACQUIRED UPON THE EXERCISE OF THIS WARRANT HAVE
NOT BEEN REGISTERED UNDER: (A) THE SECURITIES ACT OF 1933, AS AMENDED, IN
RELIANCE UPON THE EXEMPTIONS FROM REGISTRATION PROVIDED IN SECTIONS 3 AND 4 OF
SUCH ACT AND/OR REGULATIONS D PROMULGATED THEREUNDER; OR (B) ANY STATE
SECURITIES LAWS IN RELIANCE UPON APPLICABLE EXEMPTIONS THEREUNDER. THESE
SECURITIES MUST BE ACQUIRED FOR INVESTMENT ONLY FOR THE ACCOUNT OF THE INVESTOR
AND MAY NOT BE SOLD OR TRANSFERRED IN THE ABSENCE OF SECURITIES LAWS OR AN
OPINION OF COUNSEL ACCEPTABLE TO THE CORPORATION OR ITS REPRESENTATIVES THAT
SUCH SALE OR TRANSFER WOULD NOT VIOLATE APPLICABLE SECURITIES LAWS OR
REGULATIONS.

WARRANT NO.  1                                             TO PURCHASE
                                                          SHARES OF COMMON STOCK
                                                                (NO PAR VALUE)

                      CLASS "T" WARRANT TO PURCHASE SHARES
                               OF COMMON STOCK OF
                                  PROBEX CORP.
                            (A COLORADO CORPORATION)

                         PURCHASE PRICE PER SHARE: $0.50

EXPIRATION DATE: 5:00 P.M., DALLAS, TEXAS TIME, ON

         THIS CERTIFIES that, for value received,

is the registered owner and is entitled, subject to the terms and conditions of
this Warrant, until the Expiration Date, to purchase the number of shares set
forth above of the Common Stock, no par value (the "Common Stock"), of Probex
Corp. (the "Corporation") from the Corporation at the purchase price set forth
above. The number of shares of Common Stock which may be received upon the
exercise of the Warrants and the price to be paid for each share of Common Stock
are subject to adjustment from time to time as hereinafter set forth.

1. EXERCISE OF WARRANTS. Subject to the provisions hereof, this Warrant may be
exercised in whole or in part until the Expiration Date, by delivery of this
Warrant to the Corporation with the exercise form duly executed and payment of
the purchase price (in cash or by certified or bank cashier's check made payable
to the order of the Corporation) for each share purchased.

2. CORPORATION'S COVENANTS AS TO COMMON STOCK. Shares deliverable on the
exercise of this Warrant shall, at delivery, be fully paid and non-assessable,
free from taxes, liens, and charges with respect to their purchase. The
Corporation shall at all times reserve and hold available sufficient shares of
Common Stock to satisfy all conversion and purchase rights of outstanding
convertible securities, options and warrants.

3. METHOD OF EXERCISE; FRACTIONAL SHARES. The purchase rights represented by
this Warrant are exercisable at the option of the registered owner in whole at
any time, or in part, from time to time, within the period above specified,
provided, however, that purchase rights are not exercisable with respect to a

<PAGE>   2

Class "T" Warrant No. 1                                                   Page 2

fraction of a share of Common Stock. In lieu of issuing a fraction of a share
remaining after exercise of this Warrant as to all full shares covered hereby,
the Corporation shall either (1) pay therefor cash equal to the same fraction of
the then current Warrant purchase price per share or, at its option, (2) issue
scrip for the fraction, in registered or bearer form approved by the board of
directors of the Corporation, which shall entitle the holder to receive a
certificate for a full share of Common Stock on surrender of scrip aggregating a
full share. Scrip may become void after a reasonable period (but not less than
six months after the expiration date of this Warrant) determined by the board of
directors and specified in the scrip. In case of the exercise of this Warrant
for less than all the shares purchasable, the Corporation shall cancel the
Warrant and execute and deliver a new Warrant of like tenor and date for the
balance of the shares purchasable.

4. ADJUSTMENT OF SHARES PURCHASABLE. The number of shares purchasable hereunder
and the purchase price per share are subject to adjustment from time to time as
specified in this Warrant.

5. REDEMPTION. The Corporation has no redemption rights pursuant to this
Warrant.

6. LIMITED RIGHTS OF OWNER. This Warrant does not entitle the owner to any
voting rights or other rights as a shareholder of the Corporation, or to any
other rights whatsoever except the rights herein expressed. No dividends are
payable or will accrue on this Warrant or the shares purchasable hereunder
until, and except to the extent that, this Warrant is exercised.

7. EXCHANGE FOR OTHER DENOMINATIONS. This Warrant is exchangeable, on its
surrender by the registered owner to the Corporation, for new Warrants of like
tenor and date representing in the aggregate the right to purchase the number of
shares purchasable hereunder in denominations designated by the registered owner
at the time of surrender.

8. TRANSFER. Except as otherwise above provided, this Warrant is transferable
only on the books of the Corporation by the registered owner in person or by
attorney, on surrender of this Warrant, properly endorsed. However, because this
Warrant has not been registered under the Securities Act of 1933, as amended,
and applicable state securities laws, this Warrant may not be sold or
transferred in the absence of an effective registration of it under such Act and
all other applicable securities laws or an opinion of counsel acceptable to the
Corporation or its representatives that such sale or transfer would not violate
applicable securities laws or regulations. Any Common Stock purchased upon
exercise of this Warrant shall also be subject to the same restrictions on
transfer and will contain the same transfer legend found on the face of this
Warrant.

9. RECOGNITION OF REGISTERED OWNER. Prior to due presentment for registration of
transfer of this Warrant, the Corporation may treat the registered owner as the
person exclusively entitled to receive notices and otherwise to exercise rights
hereunder.

10. EFFECT OF STOCK SPLIT, ETC. If the Corporation, by stock dividend, split,
reverse split, reclassification or shares, or otherwise, changes as a whole the
outstanding Common Stock into a different number or class of shares, then:

         (1) the number and class of shares so changed shall, for the purposes
of this Warrant, replace the shares outstanding immediately prior to the change;
and

         (2) the Warrant purchase price in effect, and the number of shares
purchasable under this Warrant, immediately prior to the date upon which the
change becomes effective, shall be proportionately adjusted (the price to the
nearest cent). Irrespective of any adjustment or change in the Warrant purchase
price or the number of shares purchasable under this or any other Warrant of
like tenor, the Warrants theretofore and thereafter issued may continue to
express the Warrant purchase

<PAGE>   3

Class "T" Warrant No. 1
                                                                          Page 3

price per share and the number of shares purchasable as were expressed in the
Warrants when initially issued.

11. EFFECT OF MERGER, ETC. If the Corporation consolidates with or merges into
another corporation, the registered owner shall thereafter be entitled on
exercise to purchase, with respect to each share of Common Stock purchasable
hereunder immediately before the consolidation or merger becomes effective, the
securities or other consideration to which a holder or one share or Common Stock
is entitled in the consolidation or merger to assure that all the provisions or
this Warrant shall thereafter be applicable, as nearly as reasonable may be, to
any securities or other consideration so deliverable on exercise of this
Warrant. The Corporation shall not consolidate or merge unless, prior to
consummation, the successor corporation (if other than the Corporation) assumes
the obligations of this section 11 by written instrument executed and mailed to
the registered owner at the address of the owner on the books of the
Corporation. A sale or lease of all or substantially all the assets of the
Corporation for a consideration (apart from the assumption of obligations)
consisting primarily or securities is a consolidation or merger for the
foregoing purposes.

12. NOTICE OF ADJUSTMENT. On the happening of an event requiring an adjustment
of the Warrant purchase price or shares purchasable hereunder, the Corporation
shall forthwith give written notice to the registered owner stating the adjusted
Warrant purchase price and the adjusted number and kind of securities or other
property purchasable hereunder resulting from the event and setting forth in
reasonable detail the method of calculation and the facts upon which the
calculation is based. The board of directors of the Corporation, acting in good
faith, shall determine the calculation.

13. NOTICE AND EFFECT OF DISSOLUTION, ETC. In case a voluntary or involuntary
dissolution, liquidation, or winding up of the Corporation (other than in
connection with a consolidation or merger covered by Section 11 above) is at any
time proposed, the Corporation shall give at least 10 days' written notice to
the registered owner prior to the record date as of which holders of Common
Stock will be entitled to receive distributions as a result of the proposed
transaction. Such notice shall contain: (1) the date on which the transaction is
to take place; (2) the record date as of which holders of Common Stock will be
entitled to receive distributions as a result of the transaction; (3) a brief
description of the transaction; (4) a brief description of the distributions to
be made to holders of Common Stock as a result of the transaction; and (5)an
estimate of the fair value of the distributions. On the date of the transaction,
if it actually occurs, this Warrant and all rights hereunder shall terminate.

14. METHOD OF GIVING NOTICE; EXTENT REQUIRED. Notices shall be given by first
class mail, postage prepaid, addressed to the registered owner at the address of
the owner appearing in the records of the Corporation. No notice to warrant
holders is required except as specified in Sections 12 and 13.

15. ACCESS TO INFORMATION. The Company will provide an opportunity to any
registered owner of this Warrant to ask questions of management of the Company
and to obtain information to the extent the Company has the same in its
possession prior to any exercise of the owner's rights to purchase Common Stock
under this Warrant. Requests for information and any other questions concerning
the business and affairs of the Company should be directed to any officer of the
Company at its main offices.
<PAGE>   4

Class "T" Warrant No. 1
                                                                          Page 4

         Witness the seal of the Corporation and the signatures of its
authorized officers.

Date______________________(Seal)                     PROBEX CORP.

ATTEST:

-----------------------------------
                 Thomas G. Plaskett
Chief Executive Officer & President

<PAGE>   5

Class "T" Warrant No. 1
                                                                          Page 5

                                  TRANSFER FORM

         For value received, the undersigned hereby sells, assigns, and
transfers to

Name__________________________________

Address_________________________________

this Warrant and irrevocable appoints ___________________ attorney (with full
power of substitution) to transfer this Warrant on the books of the Corporation.

Date:

-----------------------------------------

                                           -------------------------------------
                                                    (Please sign exactly as name
                                                             appears on Warrant)

                                           Taxpayer ID No.______________________

In the presence of                         Signature guaranteed by

-----------------------------              -------------------------------------

<PAGE>   6

Class "T" Warrant No. 1
                                                                          Page 6

                                  EXERCISE FORM

         The undersigned hereby: (1) irrevocably subscribes for ________________
shares of your Common Stock pursuant to this Warrant, and encloses payment of
$____________________ therefor; (2) requests that a certificate for the shares
be issued in the name of the undersigned and delivered to the undersigned at the
address below; and (3) if such number of shares is not all of the shares
purchasable hereunder, that a new Warrant of like tenor for the balance of the
remaining shares purchasable hereunder be issued in the name of the undersigned
and delivered to the undersigned at the address below.

Date:

____________________________

                                        ________________________________________
                                                (Please sign exactly as name
                                                  appears on Warrant)

                                        Address:________________________________

                                                ________________________________

                                        Taxpayer ID No._________________________

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