I couldn't find the exact torque, but I want to warn you about another
issue, and maybe get an advice from other members. I don't have much
experience with working on cars, but I did use the extension with the
special plug socket. Sometimes the grip between the socket and the plug is
a little stronger than the grip between the socket and the extension. In
that case the plus socket will be left deep down after you finish
tightening the plug.
One way to get it out would be to just unscrew the plug again.I have a
socket with a rotating arm, which bent down after the extension was pulled
and I couldn't reattach it. Luckily I was able to fish it out with a hook
that grabbed the joint. Later I tied an electric cord to the joint and now
I can pull it out.
Probably other members would have better solutions to avoid loosing the
socket, but I just wanted to warn you about it. After you place the plug
in the socket, connected to the extension, try to pull the plug out and
see if you also have the socket in your hand.