The owner of ORCAN engines told me that plugs need to be fresh/new. If a plug is in the package for more than 6 months--the life and performance of the plug decreases a lot. He also told me that Nova changed their manufacturing for the better in early 2012. He is a big fan of the nova C7GTF plug for .12 engines.
Since getting fresh/new C7GTF plugs (for 4-5 months now), running in tropical (hot and humid) weather, I have had my best lap times, my plugs now last 2-3 times as long as they used to and there is no noticeable difference in power over the 6F.
I am a believer.
Sadly, it is hard to tell the manufacture date of plugs so buy from someone who you trust and who gets them as direct as possible from Nova.
do you mean the filament will oxidized if keep it too long