Am thinking you meant a different unit of measure than 5 lbs. per gallon of gasoline (0.5 grams?). A gallon of liquid is a little over 8 lbs., so, a material like sulfur dioxide in the gas would be a tiny amount of that 8+ lbs.
Perhaps you can put the correct amount in when you get a chance?
Factoid, it's in your wine too!
http://www.piwine.com/use-and-measurement-of-sulfur-dioxide-in-wine.html