The Silverwing's ignition system uses only 1 coil to fire both plugs at the same time. That means both plugs fire together every revolution of the crankshaft. There will be a wasted spark in each cylinder on the exhaust stroke of each. Granted, the wasted spark accounts for almost no wear of the electrodes, as the flow of electrons does not have to battle the high cylinder pressure which is absent on the exhaust stroke. But the double sparking of each plug as well as the higher rpm operation vs the Goldwing engine could account for the 8k mile recommendation. I have seen plugs come out of the Swing engine at the 8k interval and they have been out of spec with too much clearance. I just Googled, and I see that the GL1800 uses 3 coils. That means it works the same as the Silverwing ignition system. So I will chalk up the extended life of the Gwing plugs to the engine's much lower rpm operation and lower cylinder pressures due to its lower compression ratio.