From Permatex:
Nickel:
Protects metal parts from seizing and galling at temperatures up to 2400 degree fahrenheit
For use with stainless steel, titanium and nickel alloys
For use under conditions of extreme pressure and temperature
Recommended where copper contamination must be avoided
Suggested Applications: Exhaust manifold bolts, exhaust system bolts, muffler clamps and tailpipe assemblies
Copper:
Prevents seizing, corrosion and galling where high temperature conditions exist
Fortified with high quality rust and corrosion inhibitors
Temperature range: -30F to 1800F
Provides good electrical conductivity
Suggested Applications: Spark plug threads installed in aluminum, exhaust manifold bolts, engine bolts, oxygen sensors, knock sensors, thermostat housing bolts, fuel filter fittings and battery cable connections
Standard:
Highly refined blend of aluminum, copper and graphite lubricants
Salt, corrosion and moisture resistant, this lubricant is ideal for marine use
Prevents galling, corrosion and seizing and to assure easier disassembly
Suggested Applications: For easy removal of spark plugs, cylinder head and exhaust head bolts
Temperature range: -60F to 1600F
There are also non-metallic versions that just use graphite...