I find that OS plugs... especially the 8 have made my engines idle extremely well. While the difference wasn't as huge as you found, but it was still noticeable that the OS kept it idling much smoother.
If you run too rich, what happens is oil builds up on the plug. Too much and the plug will become fouled up and run just as innefficiently as a plug run in overly lean conditions. A plug lasts longest in an engine that has been properly tuned everytime it has been run.