Does a fl oz and an oz way the same?

No they are not.

A fluid ounce is a measure of liquid volume while an ounce is a measure of mass or weight.

A fluid ounce of milk and a fluid ounce of tar do not weigh the same.

No they do not weight the same. A fluid ounce is a measure of volume, how much space it takes up. Weight is a factor of density and mass. Think of having a glass of mercury and a glass of water. Both have the same volume, but their weight will be very different!