For now the Oracle Writer isn't capable of controlling the width between bytes and characters for varchar fields. The writer forwards the entered width to the database and the database uses the NLS_LENGTH_SEMANTICS settings to create a field either in byte or char length.
A writer parameter to "control" the width (NLS_LENGTH_SEMANTICS) would be a great improvement and if it's not a big deal this settings could be integrated into the Writer feature type.
Oracle it self recommends to set the NLS_LENGTH_SEMANTICS parameter to byte (see https://docs.oracle.com/cd/E24693_01/server.11203/e24448/initparams149.htm). If the user wants to use char he has to use a workaround with "SQL To Run Before Write" and "SQL To Run After Write".