Currently syslog-ng's config lists all sources explicitly. It should use system() and internal() instead, to be closer to the upstream default.

system() also adapts better to the running system – it automatically chooses between the journald socket and /dev/log depending on whether systemd is running (in case the user uses runit or other junk), and automatically uses /dev/kmsg if the kernel supports it over /proc/kmsg.

For this, scl.conf needs to be @include'd.

@version: 3.5
@include "scl.conf"
source s_local { system(); internal(); };

The upstream default is documented at: http://www.balabit.com/sites/default/files/documents/syslog-ng-ose-3.5-guides/en/syslog-ng-ose-v3.5-guide-admin/html/chapter-quickstart.html

system() is documented at:
http://www.balabit.com/sites/default/files/documents/syslog-ng-ose-3.5-guides/en/syslog-ng-ose-v3.5-guide-admin/html/configuring-source-system.html