Nah, the ounce is a unit of weight and volume. 55 gallons = 7040 fl oz ~= 7040 oz ~= 440 lbs = 200kg.

Aren't American units great? Here's a nice converter for various things, including mayo, which is ~0.94 oz per fl oz. But generally speaking, a fluid ounce is roughly the same as a weight ounce, kind of like how a milliliter is roughly the same as a gram (exactly equal for water, while a fluid ounce isn't exactly equal to a weight ounce for water).