My 350z I put 4.7 quarts in when I changed every 3500 miles with amsoil syn and I barely lost a drop... It filled back up 4.5-4.6 quarts when I drained.... Amsoil has a little meter on the side so its easy to tell this...

Do you change your own oil? If not I wonder if whomever filled it up topped off at around 4.5 and since you went longer maybe you roughly burned 1 quart every 2500 miles?