Question: is fl oz smaller than oz?
Answer: Think of one cup of flour and one cup of tomato sauce; they both occupy the same volume of space (i.e., 8 fluid ounces), but they have very different weights (about 4 ounces for the flour, and about 7.9 ounces for the tomato sauce). So no, fluid ounces and ounces should not be used interchangeably.