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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2005-12-09</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>2005-12-09</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F2"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>16.723</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>81.008</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>1988-08-08</value>
            </exerciseDate>
            <expirationDate>
                <value>1988-08-08</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>16.723</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>612.005</value>
                    <footnoteId id="F3"/>
                </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>2005-12-09</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F4"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>16.314</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>81.008</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>1988-08-08</value>
            </exerciseDate>
            <expirationDate>
                <value>1988-08-08</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>16.314</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>873.679</value>
                    <footnoteId id="F5"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F2">Dividend equivalents credited as restricted share units acquired pursuant to the Sunoco, Inc. Directors' Deferred Compensation Plan, in transaction exempt under Rule 16b-3(d).</footnote>
        <footnote id="F4">Crediting of dividend equivalents on voluntary deferred compensation account pursuant to the Sunoco, Inc. Directors Deferred Compensation Plan, in transactions exempt under Rule 16b-3(d).</footnote>
        <footnote id="F5">Total 6,624.295 Phantom Stock Units beneficially owned following reported transaction.  Reported items reflects the 2-for-1 split of Sunoco, Inc.'s Common Stock effectuated on August 1, 2005.  This stock split resulted in a doubling of the shares of Sunoco Common Stock, and related derivative securities, previously reported as being benefically owned by the reporting person.  Also, as a result of the split, the trading price of Sunoco Common Stock and the exercise or conversion price of related derivative securities was effectively halved.</footnote>
        <footnote id="F1">The conversion rate is 1 for 1.</footnote>
        <footnote id="F3">Total of 6,790.243 Restricted Share Units beneficially owned following reported transaction.  Reported item reflects the 2-for-1 split of Sunoco, Inc.'s Common Stock effectuated on August 1, 2005.  This stock split resulted in a doubling of the shares of Sunoco Common Stock, and related derivative securities, previously reported as being beneficially owned by the reporting person.  Also, as a result of the split, the trading price of Sunoco Common Stock and the exercise or conversion price of related derivative securities was effectively halved.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>John J. DiRocco, Jr., Attorney-in-fact for Robert J. Darnall</signatureName>
        <signatureDate>2005-12-09</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0202
4
2005-12-09
0000095304
SUNOCO INC
SUN
0001178443
DARNALL ROBERT J
1735 MARKET STREET
PHILADELPHIA
PA
19103-7583
1
0
0
0
Deferred Share Units
2005-12-09
4
A
0
16.723
81.008
A
1988-08-08
1988-08-08
Common Stock
16.723
612.005
D
Phantom Stock Units
2005-12-09
4
A
0
16.314
81.008
A
1988-08-08
1988-08-08
Common Stock
16.314
873.679
D
Dividend equivalents credited as restricted share units acquired pursuant to the Sunoco, Inc. Directors' Deferred Compensation Plan, in transaction exempt under Rule 16b-3(d).
Crediting of dividend equivalents on voluntary deferred compensation account pursuant to the Sunoco, Inc. Directors Deferred Compensation Plan, in transactions exempt under Rule 16b-3(d).
Total 6,624.295 Phantom Stock Units beneficially owned following reported transaction.  Reported items reflects the 2-for-1 split of Sunoco, Inc.'s Common Stock effectuated on August 1, 2005.  This stock split resulted in a doubling of the shares of Sunoco Common Stock, and related derivative securities, previously reported as being benefically owned by the reporting person.  Also, as a result of the split, the trading price of Sunoco Common Stock and the exercise or conversion price of related derivative securities was effectively halved.
The conversion rate is 1 for 1.
Total of 6,790.243 Restricted Share Units beneficially owned following reported transaction.  Reported item reflects the 2-for-1 split of Sunoco, Inc.'s Common Stock effectuated on August 1, 2005.  This stock split resulted in a doubling of the shares of Sunoco Common Stock, and related derivative securities, previously reported as being beneficially owned by the reporting person.  Also, as a result of the split, the trading price of Sunoco Common Stock and the exercise or conversion price of related derivative securities was effectively halved.
John J. DiRocco, Jr., Attorney-in-fact for Robert J. Darnall
2005-12-09