On installation of spark plugs service manual states to use Silver Grade anti-seize which I have been using for several years but not the HD branded one.
6. Apply a light coat of SILVER GRADE ANTI-SEIZE (Part
No. 98960-97) to the spark plug threads. Install and tighten
to 12-18 ft-lbs (16.3-24.4 Nm).
What I am using is the Anti-Seize 37617 also in a Silver Grade.
Typical Applications: For easy removal of spark
plugs, cylinder head and exhaust head bolts.
There is also another Anti-Seize 37616 in a Copper Grade.
Typical Applications: Spark plug threads installed
in aluminum, exhaust manifold bolts, engine bolts,
thermostat housing bolts, oxygen sensors, battery
cable connections.
Do you see an advantage in using the Copper grade over the Silver Grade, I am thinking of conductivity between spark plugs and cylinder head threads.
Both are suitable for spark plugs except the copper is for spark plugs screwed into aluminum.