OK, I'm sure I learned this in driver's ed or something, but I don't know it, and I'm a girl.

If you change your oil, does your gas mileage get better?

Yes and no. Dirty oil is always bad, but it affects other things much worse than gas mileage. Driving too fast, too much stop and go traffic, slamming on your brakes, low tires pressure, bad spark plugs...those affect gas mileage more that oil.

well, in the context of if you were to never change your oil you'd eventually get 0 miles to the gallon,
then yes.