I have read in some car workshop manuals that threads of screw should be clean and dry before fixing, but I have read one thread on here about applying copper grease onto the thread of a glow plug. Could this affect how tight the glow plug would be tightened even if done up to a torque setting? I don't want the plug to seize in the cylinder so it's near impossible to get out at another time, but also don't want to strip the thread in the alloy cylinder head.