In South Florida where it's 90deg+ with humidity that tops the charts in the summer, I've been using Novarossi C6G plugs in my .12 and OS #9 in my .21 with good results on both. Consistent running, good idle, no problems to report.
I only run 20% because I want to keep one fuel for everything and I do not race competitively. Still, I've found these plugs last a long time for me.