Amount of heat which Honda CG 125 produces in full peak summers in Pakistan, I suggest 20W50 is better in summers, IF you have to make long runs and long distance journeys. If you would be running its motor at higher revs for long periods in Summers, 20W50 would provide better cooling of Piston/Rings and cylinder walls.
Although , 20W40 is equally good and also recommended by Honda which would be good enough as it has the same ability as 20W50 when the engine is colder, but at max running engine temprerature 20W50 will have more viscosity(thicker in flowing pattern over internal parts).
As recommended by others who have used mix of different oils, they suggest for 20W50.
Additionally for Motorbikes in Pakistan, we should go for oils which are API/SG rated for wet clutch type engines (clutch works dipped inside motor oil)and even better if they are JASO MA/MA2 specified.