<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2008-06-25</periodOfReport>

    <issuer>
        <issuerCik>0000095304</issuerCik>
        <issuerName>SUNOCO INC</issuerName>
        <issuerTradingSymbol>SUN</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001178443</rptOwnerCik>
            <rptOwnerName>DARNALL ROBERT J</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1735 MARKET STREET</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>PHILADELPHIA</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>19103-7583</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Deferred Share Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2008-06-25</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F2"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>6797.17</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>39.653</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F3"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>6797.17</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3243.963</value>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Phantom Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2008-06-25</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F2"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>4918.882</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>39.653</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F3"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>4918.882</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3019.435</value>
                    <footnoteId id="F5"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F3">Not Applicable</footnote>
        <footnote id="F1">Conversion rate is 1 for 1.</footnote>
        <footnote id="F5">Total of 3,019.435 Phantom Stock Units beneficially owned following reported transaction under Sunoco, Inc. Directors' Deferred Compensation Plan I.</footnote>
        <footnote id="F2">On June 25, 2008, payment of: (a) a portion of the account balance amount deferred in the form of Share Units under the Sunoco, Inc. Directors' Deferred Compensation Plan I; and (b) all of the account balance amount deferred in the form of Share Units under the Sunoco, Inc.'s Directors' Deferred Compensation Plan II.  Each Share Unit to be paid out in cash will be valued at the average closing price of a share of Sunoco, Inc. common stock for the period of ten (10) trading days immediately prior to June 25, 2008.</footnote>
        <footnote id="F4">Total of 3,243.963 Restricted Share Units beneficially owned following reported transaction   under Sunoco, Inc.'s Directors' Deferred Compensation Plan I.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>John J. DiRocco, Jr., Attorney-in-fact for Robert J. Darnall</signatureName>
        <signatureDate>2008-06-25</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0303
4
2008-06-25
0000095304
SUNOCO INC
SUN
0001178443
DARNALL ROBERT J
1735 MARKET STREET
PHILADELPHIA
PA
19103-7583
1
0
0
0
Deferred Share Units
2008-06-25
4
M
0
6797.17
39.653
D
Common Stock
6797.17
3243.963
D
Phantom Stock Units
2008-06-25
4
M
0
4918.882
39.653
D
Common Stock
4918.882
3019.435
D
Not Applicable
Conversion rate is 1 for 1.
Total of 3,019.435 Phantom Stock Units beneficially owned following reported transaction under Sunoco, Inc. Directors' Deferred Compensation Plan I.
On June 25, 2008, payment of: (a) a portion of the account balance amount deferred in the form of Share Units under the Sunoco, Inc. Directors' Deferred Compensation Plan I; and (b) all of the account balance amount deferred in the form of Share Units under the Sunoco, Inc.'s Directors' Deferred Compensation Plan II.  Each Share Unit to be paid out in cash will be valued at the average closing price of a share of Sunoco, Inc. common stock for the period of ten (10) trading days immediately prior to June 25, 2008.
Total of 3,243.963 Restricted Share Units beneficially owned following reported transaction   under Sunoco, Inc.'s Directors' Deferred Compensation Plan I.
John J. DiRocco, Jr., Attorney-in-fact for Robert J. Darnall
2008-06-25