Is there any diffeence between Denso iriduim plug and a NGK iridium plug, or are they pretty much the same. I do perfer NGK's over other plugs that i have tried. My other question is i checked my truck plugs (8 cylinder) and have a tan color on the insulator is that normal on 30000km on a irdium plug. My previous vechicle 4 clinder i check the plugs (iridium) at 70000 km and didn't have a tan color. Should i be thinking about changing the plugs, yet or wiat until 100000 km's.