Always use factory recomended gaps unless the engine has been modified. The manufacturer has designed the complete ignition system as a package; the voltage and timing of the spark from the ignition system is tailored for a specific plug gap. Changing the gap can lead to intermittent misfirng, which will foul the plug.
When altering the gap size for high performance ignition modifications, never go more than +/- 0.008" from the stock setting because the gap surfaces between the ground and the center electrode may not be parallel for larger changes. Higher compression or boosted engines generally require a smaller plug gap and a higher ignition voltage
