Which Oil weight would be suitable for peak performance city driving conditions + max mpg, respectively for a Diesel Vehicle ?

5w30/40 Vs. 10w40 Vs. 15w40 Vs. 20w50

Mainly market competitors & suitable oils examples liked by public in pakistan market:
1. Shell Rimula r4 15w40
2. Delo Gold Ultra 15w40
These two oils are well-known for heavy usage & beating. but city driving does'nt requires that much beating.

some international consumers consider Petrol version of OILS for city driving, also light off-roading, example:
1. Shell Helix
2. Caltex Havoline
these OILs in Pakistan are known to use for PETROL vehicles only.
but both these oils are also API SN/CF, if you read the details at the back of their respective boxes , which means it is suitable for Diesel vehicles, as well as they are described in their details as consumed by blow-by re-circulation vehicles with turbo & naturally aspirated vehicles etc etc, everything mentioned there. Despite all these detailed specs, people still say & the guys at the pump claim its only for PETROL vehicles.

There is lots of confusion going on, which OIL is precisely & accurately to be used for the respective vehicle to get the most out of it, which includes peak performance with max mpg as well as good engine lubrication.

Temperature conditions:
Not less than 0 Degree Celsius max ; Not more than 40 Degree Celsius max

City driving conditions mainly include:

1. Stop-n-go traffic
2. A/C running continuously
3. Short trips
4. Longer idling

Need everyone to share their respective experiences and knowledge.
Drive Safe.
ALLAH BLESS Everyone ! (Ameen )