This document (https://help.ubuntu.com/community/NetworkManager0.7 - in section titled 'Creating/Modifying a User connection') has an interesting claim:
Hint Auto created connections are not editable. Modifying them without changing names will not be saved
Yet to confirm with a reboot, but custom DNS servers remained in /etc/resolv.conf after a variation on this sequence:
- Edit Connections...
- Edit 'Auto eth0'
- Change to Automatic (DHCP) addresses only in 'IPv4 Settings' tab
- Added DNS Servers 188.8.131.52, 184.108.40.206
- Change 'Auto eth0' to 'DHCP+static-DNS' (arbitrary name) and 'Close'
- Verify /etc/resolv.conf contains custom DNS servers
- Uncheck 'Enable Networking' to disable networking and verify /etc/resolv.conf empty
- Re-check 'Enable Networking'