Most of the candidates who attempted this question could not give the required responses to part (a) and (b) of the question.  The required response to part (a) is:  Multigrade oils are oils which contain additives that reduce the extent to which they thin out when heated and thicken when cooled.  They make cold starting very easy and also provide adequate lubrication at high operating temperature.

The required response to part (b) is:  The difference is that SAE 20 becomes thick when cold and thin when hot whereas the thickness of SAE 20/20W remains the same when hot or cold.