You can use stock plugs if it is tuned right. I used stock heat range plugs on my car for years, because I was conservative on the tune. I just switched to one heat range colder than stock, and to be honest, I really don't like them as much but went with them because it seems like the thing to do. I am using Autolite AR3924's with the cut-back tip.
Seems like the colder plugs cause the idle to hang longer than stock heat range plugs before it comes back down.
Tune it right with a safe air/fuel and you don't 'have' to use colder plugs. It is a tad safer, however.
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