If the densities of both liquids are the same, that's true. But gas and oil do not have the same density. To illustrate, you've probably heard the old elementary riddle of which weighs more, a pound of lead or a pound of feathers, right? So if we were mixing them, and I gave you a ratio of 1 gallon of feathers to one gallon of lead, you'd screw it up royally if you decided to switch to weight as your measurement.
It's true that the user can change to a different volumetric measurement and be ok. 128oz are in a gallon, so one can calculate the actual ratio and then use it for any unit they want. Or if you know what the mix is supposed to be, say 40/1, any volumetric unit will work. But you can't switch to a weight unit unless you recalculate what the ratio is supposed to be using weight measures.