This is the second winter that I've had my 6 and I've noticed that when the climate gets colder, my gas mileage drops off significantly. During the spring and summer months, I get between 350-360 miles on a tankful. During the winter months, I only get 300 miles. Has anybody else experienced this as well? I've heard the oil companies put additives in the gas during winter, which might affect mileage, but I think a 50 mile dropoff is a bit much.