Bought my 2010 fj recently with 16K miles. Lady driven, in town mostly. Still has original plugs. I have put 5K miles on it, so I now have 21K and it runs great. I know milage wise the plugs should be good for a good while yet, but should the time and city driving for 9 years have any detrimental effect on them? Runs great, fuel milage 15 to 19 mpg, but could new plugs make it better?