Yes if you want to get scientifically technical about it and make a easy answer that explains the situation we were talking about into a hard to give answer; you are correct. Different kinds of oil weigh different amounts but most fall in the 7 lb per gallon range. Heck weighing oil at night will give you a different weight than if you weigh it during the daytime.
So now would you like to talk about how when a refinery sells a gallon of diesel or gas they sell a gallon. But by the time it gets to you putting this same gas or diesel in your truck you are paying for a gallon but not really getting a gallon. You may get a little more or a little less but you very rarely ever get right at a gallon of fuel. And it has nothing to do with how the pump is calibrated.