When I was changing my spark plugs I noticied one (only 1) plug had oil on the threads. No oil on the insulator area just the threads. Is this common? I've never seen it before.
You could have had some liquid in the bottom of your plug well that got on the threads as you backed it out. You do not really read a plug by looking at the threads. Here's an article for you to read: http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html