A common mistake is to think viscosity as a quality parameter. That's wrong. Oil viscosity is a physical, not a quality, feature that represents the internal resistance of a fluid to flowing. This property explains in which temperature range the product can work properly. Let's put it simple. In working conditions the more an engine oil keeps its viscosity parameters unchanged (at low and high temperatures alike) the more the product is stable and quality focused. If your engines warm up too much there might be several reasons. One of them is engine oil. You might have picked the wrong viscosity for that engine or you might either have picked the correct one of a low quality product which lost its physical properties. 

Yes, synthetic or fully synthetic engine oils are made with oil bases with higher performance potentials. What end users usually don't consider maybe their car doesn't require that. Let's make an example. Classic car engine oils are made with less aggressive chemistries. Different building periods usually require more delicate formulas. In this case a mineral engine oil might work even better on that car. This is a borderline example. But the point is: generalizations are dangerous. To each application there's a correct engine oil, the truth lies in the pages of your User Manual. 

Using fuel economy fluids or not isn't up to you. It's a Manufacturer's choice. What Fuel or Ultra Fuel Economy engine oils have in common is to be low viscosity fluids. Low viscosity generates less friction between components and therefore less fuel consumption. If the engine of your car wasn't conceived for low viscosity fluids the risk is to burn the bearings and undermine the engine functioning.

It is not advised to top up mixing engine oils. Let's say that in case of products with similar viscosity and specifications that is tolerated. You should totally avoid mixing products with different viscosities or different chemistry. You can do it in case of emergency with the condition to make a full engine oil change shortly after.