Well, my parents have a 99 Altima (2.4L 4spd atx) and recently I've noticed the mileage go down hill.
Last year, I drove down to Florida, going 120Km/hr and got like 175Km's per 15L's using Chevron, 155 on Sunoco. Also, the oil that was in the engine was Esso XD-3 with a Napa silver line filter...