Dave is referring to fluid onces by volume. Measure your castor by volume. This is the best way.

A gallon of fuel contains 128 fluid onces as Dave said but jug and fuel together usually weighs right around 100 onces if you set it on a scale. So if you are going to weigh your castor just use 1 oz.

In other words, a 16 oz fuel tank only holds 12 oz of fuel. This is all due to the specific gravity of fuel being around .75 that of water.

I usually just visually do it by pouring "about that much" in by eye? It is not so critical.