i drive a truck with a n14 cummins, it has allways had conventional oil in it. a week ago we fixed a seal that was letting 1 gallon of oil leak out every 800 miles.this leak never was a small leak,it went from no leak to leaking a gallon in one trip. so this means the conventional oil must have caused the seal to go bad. synthetic oils do not cause oil leakes, bad seales do.