Shane uses lucas fuel treatment in 1985 GMC Sierra

I have a 1985 GMC Sierra with the 6.2 diesel with 220,000 miles on it. I usually get 17-21 mpg I started using the lucas fuel treatment and i got 27 mpg on a 300 mile trip last weekend. I checked the mileage 3 times because i didn't believe it. Now i use it every time i fill up.