In my experience RESISTOR type plugs cause idle roughness (misfires) and increase HC. All the plugs so far mentioned are resistor types (an R in the designation), and so are Bosch Platinums.

I believe NGK makes a non-resistor plug equivalent to the H9DC, but don't remember the number. The NGK Web site should have a cross reference chart.

The OE H9DCO has a "heavy duty" (larger) electrodes, but that's no big deal. The H9DC does the job.

I recall there was a recent thread on this issue and some found the H9DC on ebay or somewhere else on the Web.

Duke