Multi Grade oils were actually invented to cater for the ambient temperatures. Ambient temperature is very very important in selecting an oil.
Actually first two to three minutes after engine start are very important as this is the time when max wear and tear is anticipated.
The engine and its oil are at ambient temperature. And its this quality of a multi grade oil that saves the engine i.e it is able to properly lubricate the engine when itself is at ambient temperature.
That is what for engine oil and car manufacturer's give temperature graphs which tell us that this oil is good for such ambient temperature range.
If ambient temperature was not important we would still be using single grade oils.