hi all! i have noticed that recently there is a small puff of blue smoke out the back of my car, sometimes it is when i stop, and others when i change gear? it doesnt do it when i cane the sh*t out of it? also the back of the car has small specks of black soot up the bumper and tailgate, like its over fueling? the exhaust is always very black. the car has 130k on the clock, and has vauxhall 10/40 semi oil in it. has any one got any ideas. (i)