Can you help with the quotation marks for this dynamic SQL query?

I've made a connected between MS SQL Server 2008 and Oracle 11.x using a linked server (provider OraOLEDB.Oracle). The table I'm querying in Oracle contains a column of type TIMESTAMP(6) therefore I'm unable to read it using a SELECT * FROM ... query in the SQL Server Management Studio.

The workaround I have it to use an OPENQUERY, which returns results, however I'd like to pass variables to this query as the table I'm querying is quite large.

This one works:

DECLARE @PRODUCT CHAR(8)
SET @PRODUCT = 67660664
DECLARE @SQL VARCHAR(500)
SET @SQL = 'SELECT * FROM OPENQUERY(LINKED_SRV, ''SELECT PRODUCT , JOUR, PRICE FROM PRICE_HISTORY WHERE PRODUCT = ' + CAST(@PRODUCT AS CHAR(8)) + ''')'
SELECT @SQL

EXEC(@SQL)

This one does not:

DECLARE @JOUR CHAR(9)
SET @JOUR= '01-APR-10'
DECLARE @SQL VARCHAR(500)
SET @SQL = 'SELECT * FROM OPENQUERY(LINKED_SRV, ''SELECT PRODUCT , JOUR, PRICE FROM PRICE_HISTORY WHERE JOUR = ' + CAST(@JOUR AS CHAR(9)) + ''')'
SELECT @SQL

EXEC(@SQL)

The reason the second one doesn't work is because the 01-APR-10 needs to be in single quotation marks whereas in the previous query product 67660664 did not. So, if I was to query the database directly both of the below would work:

SELECT PRODUCT , JOUR, PRICE FROM PRICE_HISTORY WHERE PRODUCT = 67660664
SELECT PRODUCT , JOUR, PRICE FROM PRICE_HISTORY WHERE JOUR = '01-APR-10'

I've tried adding lots of different combinations of quotations but without joy. Any ideas?