You also have to consider the density of the liquid in question. For example, a liquid ounce of water weighs considerably less than a liquid ounce of, say, mercury. Most oils are a bit less dense than water and therefore weigh less per unit volume than water, while other liquids like nitric acid and vinigar weigh more than water for the same volume measurement.
Yes, all very true. When dealing with different substances you’ve got to consider that in order to be accurate.