I think it's a perfectly logical thing for the director to do. It adds to the "a few years from now" setting and prevents the movie from seeming dated if gasoline goes higher than the price shown, which is quite possible due to failure of politics.

There is no mention of oil shortage in the original novel in written in the fifties. It does however describe how oil is transported in actual barrels on the backs of trucks. Which kind of dates it as we got rid of oil barrels a long time ago.

And why is there an "a" in the movie title? There was no "a" in the book title. They should take that "a" out of the movie title and give it to Neil Armstrong.