8 fl oz of water weighs 8 oz. The specific gravity of water is 1. A gallon of water weighs 8 lbs. A gallon of gasoline weighs 6 lbs. Gasoline is lighter than water. Oil is lighter than water. I can't tell you the specific gravity if glow fuel, but it is less than water; 8 fl oz of glow fuel weighs less than 8 oz weight.