X11 doesn't work with sudo su -

Here is the situation:

login as: pkg
Using keyboard-interactive authentication.
Password:
Last login: Tue Jan  7 22:02:07 2014 from l2554
Kickstarted on 2012-10-19
/usr/bin/xauth:  creating new authority file /home/pkg/.Xauthority

[pkg@mdc3 ~]$xterm - works
[pkg@mdc3 ~]$sudo su -

[root@mdc3 ]# xterm - doesn't work
PuTTY X11 proxy: wrong authorisation protocol attemptedWarning: This program is an suid-root program or is being run by the root user. The full text of the error or warning message cannot be safely formatted
in this environment. You may get a more descriptive message by running the
program as a non-root user or by removing the suid bit on the executable.
xterm Xt error: Can't open display: %s

Here is what you have to do
su - pkg -c "xauth list"  | xargs -n 3 xauth add


Basically we have to add the xauth created by user "pkg" to this new user "root"

[pkg@mdc3vr1138 ~]$ xauth list
mdc3vr1138/unix:10  MIT-MAGIC-COOKIE-1  d9241397a36b9ecc2cb03b07addf4008

[root@mdc3vr1138 ~]# xauth add mdc3vr1138/unix:10  MIT-MAGIC-COOKIE-1  d9241397a36b9ecc2cb03b07addf4008