whitish build up on ur plugs is caused by excessive heat, not a huge concern if ur running nitrous, if they look like that after just a little wear it might raise a flag but if those are pretty worn then they look good, no oil build up or corrosion, carbon or ash deposits, or signs of glazing or detonation, my recommendation check for the correct plug heat range, overadvanced ignition timing, lean fuel mixture, intake manifold vacume leaks, sticking valves and insufficent engine cooling.