Are you looking for a single file that will age out the oldest log lines? If so this is not something that can be done the built in functionality. If you are looking to keep 30 days in a file then create a new file for the next 30 days that is possible. We store the logs per host in daily files and deleted after 90 days. If you're looking for search capability you could try the web interface.
Also, when you say "server name at the beginning of the file" do you mean the first line in the file itself or in the filename?