What harm? Buildup of sludge inside engine since this particular vitz engine is not designed to use thick oil like 20W50.
Where do 'Pakistani conditions' conditions to this? Climate conditions have nothing to do with engine oil. 20W50 is used in Pakistan because it is recommended by manufacturer for specific engine models which are designed to use it.
Swapping a kabli engine is not solution to these issues. How does one know if the swapped engine was really great? The solution is to rebuild the existing engine with OEM parts.
If his car's engine burns oil and smokes, 20W50 is not the last option.
Nobody is criticising here, just sharing the correct advise.