the reason the brake fluid is on top of everyone's list is because
as it is in storage ,the temp of the building gets hot then cold that causing condensation,and water in the brake fluid means your brakes does not work as well ,,water in brake fluid also allows your calibers to rust from the inside out,
this is for every part in your car that contains gas, and or ,oil (water does not mix with oil

it is not the mileage ,but the temp change,, now you know why Jay leno has heat and AC wher he stores any and all his cars. jim