Exhibit 5.1

December 21, 2017

Andeavor,

    19100 Ridgewood Parkway,

        San Antonio, Texas 78259.

Ladies and Gentlemen:

In connection with the registration under the Securities Act of 1933 (the “Act”) of $500,000,000 aggregate principal amount of 3.800% Senior Notes due 2028 and $500,000,000 aggregate principal amount of 4.500% Senior Notes due 2048 (collectively, the “Securities”) of Andeavor, a Delaware corporation (the “Company”), we, as your counsel, have examined such corporate records, certificates and other documents, and such questions of law, as we have considered necessary or appropriate for the purposes of this opinion.

Upon the basis of such examination, we advise you that, in our opinion, the Securities constitute valid and legally binding obligations of the Company, subject to bankruptcy, insolvency, fraudulent transfer, reorganization, moratorium and similar laws of general applicability relating to or affecting creditors’ rights and to general equity principles.

The foregoing opinion is limited to the Federal laws of the United States, the laws of the State of New York and the General Corporation Law of the State of Delaware, and we are expressing no opinion as to the effect of the laws of any other jurisdiction.

We have relied as to certain factual matters on information obtained from public officials, officers of the Company and other sources believed by us to be responsible, and we have assumed that the Indenture under which the Securities were issued have been duly authorized, executed and delivered by the Trustee thereunder, an assumption which we have not independently verified.

We hereby consent to the filing of this opinion as an exhibit to a Current Report on Form 8-K to be incorporated by reference into the Registration Statement relating to the Securities and to the reference to us under the heading “Validity of Securities” in the Prospectus Supplement relating to the Securities, dated December 14, 2017. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Act.