Filed by Tesoro Corporation

(Commission File No. 001-03473)

Pursuant to Rule 425 under the

Securities Act of 1933, as amended

Subject Company:

Western Refining, Inc.

(Commission File No. 001-32721)

March 6, 2017

Dear Stockholder:

We previously sent you proxy material for the special meeting of stockholders of Tesoro Corporation to be held on March 24, 2017 in connection with the proposed acquisition by Tesoro Corporation of Western Refining, Inc.

Tesoro Corporation’s board of directors unanimously recommends that stockholders support the acquisition by voting FOR the issuance of Tesoro Corporation common stock in connection with the merger and FOR all other items on the agenda.

Your vote is important, no matter how many or how few shares you may own. Whether or not you have already done so, please vote TODAY by telephone, via the Internet, or by signing, dating and returning the enclosed proxy card in the envelope provided.

Thank you for your support.

Very truly yours,

Geoffrey J. Goff

Chairman, President and Chief Executive Officer

IMPORTANT NOTE:

Remember, you can vote your shares by telephone or by Internet.

Please follow the easy instructions on the enclosed proxy card.

If you have any questions, or need assistance in voting

your shares, please call our proxy solicitor,

INNISFREE M&A INCORPORATED

TOLL-FREE, at 1-888-750-5834.

No Offer or Solicitation:

This communication relates to a proposed business combination between Western and the Company. This announcement is for informational purposes only and is neither an offer to purchase, nor a solicitation of an offer to sell, any securities or the solicitation of any vote in any jurisdiction pursuant to the proposed transactions or otherwise, nor shall there be any sale, issuance or transfer or securities in any jurisdiction in contravention of applicable law. No offer of securities shall be made except by means of a prospectus meeting the requirements of Section 10 of the Securities Act of 1933, as amended.

Additional Information and Where to Find It:

This communication may be deemed to be solicitation material in respect of the proposed transaction between Tesoro and Western. In connection with the proposed transaction, Tesoro has filed with the SEC, and the SEC has declared effective, a registration statement on Form S-4 (Reg. No. 333-215080 ), containing a joint proxy statement/prospectus of Tesoro and Western, which proxy statement/prospectus was first mailed to Tesoro and Western stockholders on February 17, 2017. This communication is not a substitute for the registration statement, proxy statement/prospectus or any other documents that Tesoro or Western may file with the SEC or send to stockholders in connection with the proposed transaction. STOCKHOLDERS OF TESORO AND WESTERN ARE URGED TO READ ALL RELEVANT DOCUMENTS FILED WITH THE SEC, INCLUDING THE FORM S-4 AND THE DEFINITIVE PROXY STATEMENT/PROSPECTUS INCLUDED THEREIN, AND ANY OTHER RELEVANT DOCUMENTS FILED WITH THE SEC, BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION ABOUT THE PROPOSED TRANSACTION. Investors and security holders will be able to obtain copies of these documents, including the proxy statement/prospectus, and other documents filed with the SEC (when available) free of charge at the SEC’s website, http://www.sec.gov. Copies of documents filed with the SEC by Tesoro will be made available free of charge on Tesoro’s website at http://www.tsocorp.com or by contacting Tesoro’s Investor Relations Department by phone at 210-626-6000. Copies of documents filed with the SEC by Western will be made available free of charge on Western’s website at http://www.wnr.com or by contacting Western’s Investor Relations Department by phone at 602-286-1530 or 602-286-1533.

Participants in the Solicitation:

Tesoro and its directors and executive officers, and Western and its directors and executive officers, may be deemed to be participants in the solicitation of proxies from the holders of Tesoro common stock and Western common stock in respect of the proposed transaction. Information about the directors and executive officers of Tesoro is set forth in the proxy statement for Tesoro’s 2016 Annual Meeting of Stockholders, which was filed with the SEC on March 22, 2016, and in the other documents filed after the date thereof by Tesoro with the SEC. Information about the directors and executive officers of Western is set forth in the proxy statement for Western’s 2016 Annual Meeting of Shareholders, which was filed with the SEC on April 22, 2016, and in the other documents filed after the date thereof by Western with the SEC. Investors may obtain additional information regarding the interests of such participants by reading the proxy statement/prospectus regarding the proposed transaction when it becomes available. You may obtain free copies of these documents as described in the preceding paragraph.