10w40 for winters and 20w50 for summers..They are multigrade but still a 20w oil will take buck load of time to warm up..A 10w40 will consume less time for warming,,thus less hassles and more carefree system..
Oils are my speciality 10w20,5w20,10w30,10w40,10w50 there are many grades for different climatic conditions and engine requirements..So you need to choose your oil's viscosity by the climatic conditions you encounter..WHich results in better engine response//less wear and tear..It is a science of lubrication after all.
Masla yeh hai 20w40//20w50 oil kay saath(sardiyon main) kay cold start pay log 10-20 seconds baad scooter chalanay lagtay hain..It is just like raping your engine..20w oil ziada thick hota hai 10w ki nisbat say so isko thora time chaiyae thin honay main jo hum nai detay..result thick oil which cannot flow freely while we are over reving..
If you are using 20w40 in winters just let the engine warmup at idle rpm with some low reving in b/w in static condition..Engine will get warm but will consume a little more time so be patient