the way it was always put to me
copper is a better heatsink and cheeper, in racing you want the heat away from the plug asap (cutback groundstraps, shorter porcelin aka colder)
anbd on the street ya want to keep the heat to clean it
far better in racing to replace the plug than reuse them when reading your plugs
oil in the chamber increases preignition risks, just as much if not more is a damn glow plug (hot spot)
i just extend it to the street cause i like being able to read plugs at least once a year....it might just clue you in that an injector isnt spraying properly etc