While I wouldn't recommend anyone to arbitrarily mix fuels, unless you are tuned to a razor's edge to just one type fuel, the likelihood that you'd notice a difference if mixing fuels would be slight. The biggest thing that will degrade fuel is age and exposure to the elements- especially water.Top Tier gasolines all have proprietary detergent formulas. Is it possible/probable that a degradation of effectiveness happens if mixing between different brands occurs?
I have been in the industry most of my working life, and have learned......As the link posted above will tell you gasoline is traded openly. Your Mobil station could be using gasoline from just about anywhere. Brand loyalty is an expensive myth.