Have you guys ever used a spark plug that is hotter or colder than the original? What were the results?
Going with a colder plug during the summer months, should cause no harm. It might foul the plug with a lot of stop and go, and idling. It may be better for WOT for long periods.
A hotter plug could cause problems, for prolonged high speeds. It might be better for winter riding, and stop and go stuff.
The potential to do harm, is greater with the hotter plug. I would be hesitant to use them. If the engine is old and has low compression, which causes the plugs to foul, then you do not have much choice.
Using NGK plugs makes understanding the heat range easy. Generally a number 6 will be as hot as any manufacturer recommends.
A number 12 might be used in a track bike. For most street applications, a 7, 8, or 9 will work. But check the book. I am generally wary of a or hotter, if I am doing anything except around town puttering.
Going back to your question. You did not mention what type of engine is being used. Street bike, dirt bike, 2 stroke, 4 stroke, or how many cylinders.
UK