I have a 2005 Altima with 75K miles. It has suddenly started consuming a quart of oil every 500 miles! I have parked it over clean paper overnight: no spots. The spark plugs are clean and indicate a good combustion. There is no oil (that I can see) in the coolant. There is no oil on the block, transmission or under carriage that I can find. In a word, I'm stumped! Any one else have a similar experience? Any ideas?