If the oil doesn't mix well, it will sit on the bottom of the tank. The gasoline used first on the bottom of the tank will be rich in oil. The gasoline used later in the trip will be lean in oil. In this experiment you will determine who was right. Select a clear glass container that has a good lid.
Some plastics create static electricity with gasoline.
Pour gasoline into the container. Add the proper amount of oil to that gasoline. Do not shake it. Most oils have coloring added so it is easy to observe. Pour that gasoline into the outboard motor tank.