Its not the recommended grade Nissan designed it to use 5w30
Lots of reasons but mainly the 370z engine relies heavily on oil pressure at certain points, valve control for one, cam chain tension for another but there are many, pressure is resistance to flow , thicker oil is more resistant to flow and may increase the pressure at these points.
Generally clearances on modern engines are much better than they were and there is rarely a need to go for oil above 40w. (My V8 takes 5w20, the GT86 takes 0w20)
Also there is no need to worry about oil breaking down due to heat in the 370z , there is a temperature safeguard that limits performance if the engine temps rise above 130 degree c , synthetic oils can cope with temps over 300 degrees c for hours.
Finally if you have thick 50w oil in the sump your fuel economy and power will be reduced which is one of the reasons manufacturers have started to specify thinner oils (Emissions)