Depends on the bike: when I must depend on low-grade fuel, I add about 2 ounces of two-stroke oil when I fill up with possibly old or gummy fuel: it adds that critical bit of cylinder lube but is not enough to foul plugs -- unless you overdo it.
Using 16:1 two-stroke mix continuously can give you fouled plugs, but scant little other hassle. And if the bike has been sitting around too long, or has been run abnormally lean, or you can't get decent fuel, that two-stroke oil can save your mill.
If you somehow get iol fouling on your plugs, the easiest way to remove it is with a propane torch. Burn out the carbon but do not blue the metal parts or crack the ceramic insulation, gap per factory specs, and replace the plugs.
