The external table object of Oracle Warehouse Builder refer to the Oracle Database - External Tables of the database.
Even if you specify the '\n' separator in the file definition, the deployment of the external table replace it with the NEWLINE term and may lead if you are working with Unix file on a Windows plateform to this error :
ORA-29913: error in executing ODCIEXTTABLEFETCH callout
ORA-29400: data cartridge error
KUP-04020: found record longer than buffer size supported, 524288, in C:\PathToFile\FileToLoad.txt_
ORA-06512: at "SYS.ORACLE_LOADER", line 52
To resolve this issue, you have to modify manually this line :
RECORDS DELIMITED BY NEWLINE
by
RECORDS DELIMITED BY '\n'