Unless the mogas components are immiscible (which I doubt), once blended they will not separate. The different density components become a single fluid with approximately a weighted average density. If the mixer size is not adequate, the most likely answer to both of your questions is that the tank contents were never blended. Two layers of different density liquid may tank longer than 18 hours to blend with out sufficient mixing intensity. Blending layers of different density can be difficult. The best solution to avoid future problems is to add the denser fluid on top of the lower density liquid, or add either fluid near the mixing impeller. You want to avoid layers of different density liquid. 1) The lighter or denser components should not separate if the tank is well mixed, no matter how long they sit. 2) I do not know of any calculation, formula or simulation, that will predict separation, even for immiscible fluids. The answers by this expert are based on the best available interpretation of the information provided. The consequences of the application of this information are the responsibility of the user. If clarification is needed, please submit a further question.