Spark plugs, as long as you can’t see them why worry about them? They will last for a 100K says the manufacture. OK so they are tough to remove after that many miles but don’t worry we will sell you a special tool to remove the broken shell. Many techs are saying to give them a twist or 2 at 50K and this will prevent removal problems, or remove and clean them. On many of the newer vehicle you have a hard time seeing the plugs let alone giving them a twist or 2, also I do not believe in cleaning plugs, without special equipment it is almost impossible to remove all the crud from a plug. They are also cheap except for the platinum which they recommend not cleaning. So bottom line about 70-80K replace them. When replacing the plugs pay close attention to the insulator for carbon etchings or tracking if any are present replace the wires also. When installing the wires a touch of dielectric grease is a good idea, also remove the wire from the coil pack and place a touch there. Running a tap through the threads is also a good idea; clean threads allow more efficient heat transfer and prevents a lot of problems. This also provides an accurate torque value, and yes torque them. If you find any damage usually from preignition, this can be from an air/fuel ratio or cooling problem among others. For several years, with the advancement of ignition systems, I expected to see an advanced design replace the spark plug, but so far nothing is replacing the plug. So replace yours before they cause problems. One last hint use only the recommend plug anything thing else is going to cause problems and will not improve performance or mileage.