Well it is true that is buying Base Oil from NRL as that is the only source of Base Oil in Pakistan. Shell & Caltex are not buying oil from PSO but PSO is doing toll blending for Castrol currently and was doing for Total.
The quality of any oil depends on its performance level i.e. API level and SAE grade. You can use any engine oil that is recommended by your car manufacturer or any engine oil of same grade as described in owner's manual.