I use my van for work, hauling freight, usually 8000 miles a month. I have read where another MMO user uses one quart to a full tank of gas on a Cadillac which is a big over kill of MMO, but he claims large increase in mpg.  Usually the opposite is true, over use causes a loss in mpg, anyone have a comment.  I may try a quart to my 35 gal. tank,

Once again, the recommended dosage of Marvel Mystery Oil is 4-ounces of MMO for every 10 gallons of gasoline.  If you are using E10 or higher gasoline, then you can use 5 or 6 ounces for every 10 gallons.  More than that does not greatly improve anything due to the Law of Diminishing Returns.  Improvements in gas mileage vary greatly to engine size, type of driving, grade of gasoline, etc.