Zic 4T comes with 10/40 or 20/50 Viscosity and is of older grade like SJ or SL. These are mineral oils which gets thinner after being heated well in the engine. But yet they will perform well in the engine. So it doesn't matter either its winter or summer.
As a suggestion, buy 3L oil bottle of PTT Performa 20/50 of SM grade. This is nice oil in viscosity 20/50. If you have 125, then you can use it in 3 times. Keep the change interval as 1K only.
Or
Try SIM3 oil which is again SM Grade, 20/50 viscosity, but plus point is, that, its semi synthetic and very good. CI 1K