This question never seems to get a clear answer. Permatex has the three basic types at their website: gray/silver (1600F), copper (1800f) and Nickel (2400F). The silver PDS says its good for plugs. The copper says its good for plugs in aluminum head engines.
I just got off the phone with a Permatex "tech.", and he also did not give a clear answer. He stated that basically any of them would work o.k..
When asked which was best or why the copper specifically stated use in aluminum heads, his answer was that the copper one is "better" because of its higher heat range AND better conductivity. When asked about galvanic corrosion between copper and aluminum, he said that there is some concern about this BUT it is not a concern in this application.
I stated that I didn't think engine heads approached 1600F and he said they did. WHAT"S THE TRUTH HERE>
He then said the nickel antiseize is also an excellent choice and that is has good/better conductivity than the gray/silver.
I have always used the gray/silver product, VERY minimal amount, in both iron and aluminum heads with no "apparent" problems.
Am I doing something wrong? Should we use the copper? Or, even the nickel?
I would like to hear other opinions and real world experience.