Oil content is important, but there is no standard amount. although most engine manufacturers say 18% to be safe (since they dont know what fuel the user will be using). This number, however, is arbitrary. Even if the fuel has a high percentage of oil such as 18%, if the oil burns up it is not doing what its supposed to be doing, which is lubricating and protecting the engine.
I'm just quoting a huge article i read on fuel. I just wish the article was still around. Gives very valuable insight, especially on castor and synthetic properties, percentages, etc.
BTW, when a company gives a percentage of oil, they never specify whether it is by weight or by volume. So unless you try to isolate the oil from the fuel mixture, we dont know exactly how much oil is in the fuel itself.