These are the M8 peened copper nuts you're talking about, yeah?
Use regular ones if you like working on your car, replacing the manifold gaskets.
If you'd rather enjoy driving it then buy proper peened nuts. GSF sell them and I suspect other auto-factors would also be able to supply them. You should also use nice fat plain washers under them, too.
I've seen so many bodged jobs doen with normal M8 nuts, where they've come loose that it really counts as a corner to not cut in my book - more so on turbo motors than GTIs, but no one likes the sound of a blowing manifold, least of all the MOT man.
