No, switching from volume to weight will not give you the same actual ratio. Yes, the end goal is to have X number of molecules of oil for ever Y number for molecules of gas. The engine manufacturer worked out what that needed to be. Then they gave the end user a way to get there- volumetric measurements. They could have given us weight measurements, and you could figure out what the measurement by weight should be by measuring the density of both liquids and doing some 8th grade algebra. But there is no point in doing all of that. Use a measuring cup for your oil and fill your tank to exact gallon amounts, and you'll never have a problem.