raza bhai well i am not very expert in technical matters but would share what i have understanding.
multigrade oils were develop 50 years back as before that usualy people use thinner oils in winter and thick oils in summer.
in a 5w/40 for example 5w ( w consider to be "winter" , not weight or watt ) simply means that the oil must have a certain maximum viscosity/flow at low temperature. the lower the "w" number the better the oils cold tem/cold start performance.
the 40 means that oil must fall within certain viscosity limits at 100deg c. this is a fixed limit and all oils that end in 40 must achive these limits. once again the lower the numberthe thinner the oil, a 30 oil is thinner than a 40 oil at 100 deg c etc.
karachi is not very hot like lahore and also u have new engine so u can even use 5w/30 but in my view min 5w/40 is safe option in pakistan road conditions where u stuck in traffic for hours , with ac on and road temp at that time much more than avg.
mobil 1 is safest as its 5w/50 however u can also go for caltex fully synth 5w/40 which is also good enough.
about ur passo it must be used imported one , pls advise its oddo meter reading as i heard synth oils not good for old engines as thoes start reducing oil levels in engine.