Can you justify the 20W-50, thicker oil?
Thicker than recommended oil is only used in cars like Vitz when your engine is consuming more than normal amount of oil. It is done to avoid rebuilding the engine's weak rings and seals.
Your car engine is designed keeping specific oil in mind. Thicker oil will reduce fuel economy and thinner oil will seep through the seals and burn along fuel.
And for your second question, thicker oil will give you less average.