My father recently purchased a mint 2002 Ford Taurus with a 3.0L engine that has 270000 kms on it. Engine is in great condition not burning any oil. The previous owner has been using conventional oil but now that my father owns the car he is not sure of continuing with conventional oil or switching to either regular HM or full synthetic, he has heard that switching oils that have been used in the engine for most of its life and going to synthetic can cause seal leakage. So he's puzzled at which oil is better for an engine with such high mileage? He wants this car to last a long time.