We just bought the car and it was a rental. I have a 2005 300 C. I changed the plugs at about 95k and from the trouble that I had, I think they should have been cheched sooner. Very hard to get then out. It seems lkie they were starting to seize up. The new ones that I put in, made sure I put never seize on the threads.
I know on the 2010 300 that I'm going to need gaskits, but I'm not sure which ones.