Just thought of another possibility. Are you positive that its oil on your plug, like liquid oil or just burnt plugs? Your symptoms of hesitation at 4k and black smoke also points to your car running rich.and a black tipped plug also indicates this. Is your check engine light on? Reason I'm asking is that this problem my lie in a bad or old o2 sensor. They usually go back after a 100k miles or so. Its good practice to replace that to keep your car running in top mpg conditions.