I have a motorhome on the Workhorse chassis with the 8.1L Chevy engine. I just pulled the plugs at 7 years / 37,000 miles and they were damaged by what appears to be high heat. One plug even had a piece of the insulator cracked off of the center electrode. The plug is AC Delco 41-983. I have been told to gap the new plugs at .045 instead of .060 as in the specification in order for the plugs to run cooler. Any comment on this? Also, is platinum or iridium the better plug?