Low quality fuel is the most likely culprite. With lack of oil changes or cheap oil being second. Another possibility would be incorrectly gapped spark plugs leading to more carbon in the exhuast, but 99.9% sure that the spark plugs in a car like that would be irridium and not to be gapped. An even rarer possibility would be a bad compression ring or overall poor seal in the cylinder, but you would most likely have noticed oil burning or running poorly. Of course it is possible that it was just a bad part.