I have checked 3 RC car blends commercially available off the shelf and evaporated them off so all that was left was the oil. I was verifying what the oil content really was. If mixed by weight, the volume I ended up with would have been wrong. Now I understand this was only 3 brands out of how many, but it makes the most sense to mix by volume. You end up with just a touch more nitro because it's so much heavier than the other components (about 13% heavier than castor oil).
If I used commercially made fuels still, I'd consider emailing some of the fuel companies to find out what method they really use. If one does mix by weight, you could be getting shorted on oil or nitro.
If I'm wrong, please provide information showing such. I prefer to be properly informed.