$(1)$ What is the average speed in mm$/$s through a $4\mathrm{.}0$ mm diameter tube of $20$ C motor oil if the pressure gradient is $1900$ Pa$/$m$?(2)$ What is the average speed in mm$/$s through a $4\mathrm{.}0$ mm diameter tube of $100$ C motor oil if the pressure gradient is $1900$ Pa$/$m$?$