Re: hot plug vs cold plug???
Actually there really shouldn't be any more carbon build-up at low speeds vs higher speeds. If anything, there's more time for the combustion process to fully burn, so the carbon should (theoretically) be in smaller pieces and no longer a "sticky" ion.
IF the rings and valves are sealing properly, and there's no excess oil or anything else getting in with the mixture - and the right air fuel balance (mixture) is used there should be no carbon left over.
Best to stay with the correct range all the time, unless you have an "oil pumper." Then it's a temporary fix till it gets repairs anyway.
A word of caution - parts store, JCW etc sell "Non Fouler Adapters." DON'T!!!!!!!!!! What they are is an adapter that screws in the plug hole, then the plug screws into that. It's creating a shield around the plug, supposedly keepng the oil off. It works, the plug is able to fire better.
BUT BUT BUT --- The electrode is in it's own little chamber - it gets the mixture burning inside that little chamber, then the gasses blow out through the small hole into the rest of the chamber. Those 3000+ degree gasses blowing out that hole are like a welding torch flame aimed directly at the top of the piston. Aluminum melts at what, 800-1200 degrees? Cast iron at 1800? In short order you get nice big holes through the pistons.
It's an excellent way to stop the oil fouling of a tired engine - it hastens the major repairs.