It's not that the fuel is thicker with the oil in it, it is that per volume of fuel, there is more oil in the richer mixture, thus making less fuel present. if you took one gallon of 32:1 fuel:oil, and one gallon of 100:1 fuel:oil, the latter of the two would have more fuel per gallon of mix than the 32:1. Nothing to do with thickness, or "viscosity" as I think you are mentioning.......