Bob, this may have worked on your engine(s), but you shouldn't make a "blanket statement" for others to follow.

Every engine and installation is a law unto itself. Glow plug selection depends on interacting factors of compression ratio, porting, exhaust scavenging, nitro content and engine load to name a few. The best analogy I can give you to think of, is to relate hot and cold glow plugs as to timing advance and retarding in a spark ignition engine. Hot= advance, cold=retard, realize this example is over simplified.

In most of my engines I have gone to colder than recommend plugs to gain performance when I push them to the limit.

Do I run rich, sure if the engine wants it. How about 8oz. of 30% fuel in three minutes in a 46 size DF engine. Or 16oz. of 30% in 5 minutes in a 70heli engine mounted in a full race WM P-51 mustang. A hot plug would last only seconds in those engines, the right one can last a gallon or so.

I now only use K&B brand glow plugs. HD type (like #8 but tougher ) or HP type (cool, high nitro) or Enya #5 in a pinch(cool).