If the engine is weak and older, it probably means that the seals are older and gaps in rings and pistons etc are increased.
For your car we would suggest going for Valvoline XLD 20W-50 API SG.
For oil drain interval, our recommended ranges are :
Valvoline XLD 20W-50 API SG - 6000 kms
Valvoline Premium Conventional 10W-30 API SN - 7000 kms
Valvoline Durablend 10W-40 API SN - 8500 kms
Though the oils can extend much further, the quality of filters available in Pakistan is low so it is better to change around these ranges. Our lubricants also have special patented international standard dispersants and detergents, which will help keep your engine cleaner, run smoother and run longer.
Hope this helps.
Valvoline Pakistan Official- valvoline.pk