Hmmm ... free oil change, in exchange for f*cking up your threads??? No freakin' way. I think they're playing chicken hoping you'll flinch first and just go away. This isn't a little inexpensive mistake... I'd definitely let Nissan HQ know about this and make sure someone takes responsibility for it. Take pictures if you can, and get all your service records together to help support your case. It's guys like this that give decent mechanics a bad name...