NGK seems to be the preference. Are there other variables like Platinum, Gold,etc. Do plugs actually wear out? What goes wrong? If I pulled one plug and its nice and brown with no corrosion is it good? Where do I find the correct gap sizes? I checked CarFax before I bought my 90,000 mile vehicle. No plug changes. But it looked like regular maintenance was performed ( a plus in my mind for a one owner auto) Maybe the dealer wasn't aware?