I have been told that that the warning about the O-ring damage means don't inject after-run oil into the fuel inlet of the needle valve or carb. It is ok, in fact recommended, to put oil into the carb air inlet.

I do not think putting it in the glow-plug hole is a good idea as sooner or later you are certain to cross thread or strip the head. Also, the copper washer will eventually become so deformed that it will not seal properly.