even with a fuel injected motor, you need to let it warm all the way up before shutting it off. at minimum, it has to run until the yellow light in dash goes out. i prefer to run mine until i feel heat in both running board before i shut it down when i can. since i started doing that, i only change plugs when milage/time starts making me nervous about them. i did my 11 apex plugs around the 13000mi point as i wanted to see what they looked like and that was a ton of miles for me to put on a set of sled plugs.