I just purchased a 2004 Z (auto)with 41K miles on the clock. Completely stock, no mods. I changed the oil using Pennzoil Ultra 5w30 and P1 filter. Runs like a champ. All other fluids, including transmission fluid look good and clean. Should I still change out, especially the transmission fluid. Also the plugs are OE but are 7 years old. Should they be replaced? Any help is appreciated.