honestly Im not to sure about any noticeable viscosity differences that could be realized between Fuel/Oil mixes since the actuual amount of oil that is in the fuel is so little compared to the amount of fuel and the oil is so dissolved in the fuel, but I do know that no matter what mix ratio is being run, it is important that the carb have the correct jetting for that mix along with several other deciding factors.
I know I can run straight up 100:1 Opti2 mix or I can add about and ounce of Maxima 927 Castor to 1 gallon of my 100:1 mix and not see any difference in performance and see no difference when I check my plug color.
I add the Maxima 927 Castor product because it has Castor oil + a high grade synthetic oil, since I run my engines hard for miles at a time and because I live where it is normally hot and dry I like knowing there is just a tad more protection going into my engines.
Adding the slight bit of extra oil to my mix has never shown any signs of causing a lean run Air/Fuel mix condition, possibly because I dont jet my engines on the lean side to begin with, but I do jet them to have very little 4 stroking which produces a very light brown plug color.
I think its always a good idea on these engines to jet toward the slightly rich side rather than on the lean side, performance wont be that much difference overall but the life of the engine may be extended by keeping a bit more oil in there for lubrication and the extra fuel for keeping things a little cooler since the fuel is actually the coolant of the two.