Engine Performance problem
1999 Ford Windstar 6 cyl Two Wheel Drive Automatic 163000 miles

i recently bought a used van and had a oil change done...it has been 2 wks since the oil change and now the oil is black...i have used 2 litres and put on 1200 km since the purchase..(purchased at 162,000 km)...when i go to start the vehicle it blows out white smoke from the exhaust..(only when it has sat overnite then it is fine throughout the day)..it idles fine and i find that for a van it is fine on gas mileage..