2002 Other Jaguar Models reduced power/engine noise

i have a 2002 v6 2.5L jaguar x type, and recently i have been seeing smoke from the mufler, i hear a rettling sound from the engine, and the power has been reduced alot to the point where sometimes i cant go above 30mph.
i changed the spark plugs a few weeks ago and while i was doing that i saw oil on the piston. can u tell me what the problem is??