Viscosity will not be a fact at all in this discussion at all. Simple poor mans test is you do not have a calibrated fisher cup is you take a red bull can, take a stick pin and puncture the bottom. Use a zyliss can opener and remove the top ( tab side ) then add the handle. Now mix 1 gallon of mixed fuel as you like it and 1 gallon of straight fuel.
Now get your stopwatch and measure the time it takes for it to empty from the mixed fuel to the straight fuel the time will be the same. Also why not use the oil that is meant for the task at hand? ]
50:1 is what I use, if it is good enough for all the tree crews, landscapers, and alike it is good enough for me.