What directory and file permissions for external tables on Linux?

We are running Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production on Linux version 2.6.18-308.24.1.0.1.el5. We created a directory and external tables so we can easily load data from files. Unfortunately, we cannot find Oracle documentation that describes the correct OS user ownership and permissions for the OS directory and files. According to our security policy, the user creating the files and managing the logs, appuser, has to be different from the user running the database, oracle. We tried creating a secondary group, setting the group sticky bit, and adding both appuser and oracle as members, but that