Mpg

I was on a car cruise a couple weeks ago & was talking with a guy about mpg with different vehicles. He stated that for years he has added approx. one gal. of diesel fuel per tankful of gasoline. He says the diesel fuel helps coat the cylinders/etc & that he can pick up 4-7 mpg more depending which vehicle he's driving at the time. Has anyone heard of this or was the guy just feeding me a good line of BS? Thanks.