It would have easier to answer if you had mentioned for which vehicle and engine.
1) Yes, each engines are designed to take a specific grade of oil. Initially, fuel economy could be only difference you may see but over time it can cause premature wear to the internal parts of the engines. So, always stick to what the manufacturer recommends.
2) Yes, each engines can have different style of pumps but that doesn’t define the oil viscosity but the clearance used in engine bearings and other parts determine the recommended oil viscosity.
3) Performance not much but only the fuel immediately unless you use a widely different viscosity then it can effect the performance.
4) It is fine 0W30 or 5W30 if manufacturer recommends. Check your owner’s manual for this information.
5) Again, depends if the manufacturer recommends or not. Usually people switch to non synthetic or thicker oil as the engine gets older but I never did that. If you maintain it well, you don’t need to. If your engine is leaking oil, change the seal or gasket instead of using thicker oil or additive.
6) Not required.
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