Does BSI have any support for Oracle 18c?
Oracle 18c is completely untested with BSI at the time this is written.
According to Oracle's compatibility matrix:
It looks like the 12c Oracle client can connect to an 18c database.
So almost certainly having an 18c database with a 12c client would not be a problem (though again, it is not currently tested).
However, upgrading the clients to 18c as well would likely require a patch.