oily grey? if its oily it means ur runnin to rich ( to much oil in ur fuel ) if its carbon ( sandy bits of black ) it means u have to replace it. It will fire but not well,its pretty much stuffed. Dont try scrap it off either it doesnt help at all. Spark plugs are only 5 bucks anyway so dw.
I would think we are talking in regards to a four-stroke here -- because most haven't seen any grey looking plugs pulled from a 2-stroke -- 2s usually looking pretty dark and wet -- oily..... Yes, with a four stroke I have looked for and preferred a slight grey color over the years - and with most of my engines that were running good and not burning oil - plugs and exhaust pipes were kind of a light brown grey color... Small engines may last longer running a little richer - slightly darker looking plug and pipe... Looking at and having some understanding of our spark plug condition -- can tell us a lot -- troubleshooting ect.... PS - tailpipes - never buy a used car without putting ones finger into tail pipe checking sides for amount of oil bildup - blow by -- NOT TO BE DONE WHEN HOT .. Happy Riding from - Mountainman