Every AGA is individual and therefore they can take slightly different times to heat up.
AGA recommends the use of Class C2 Kerosene. However, suitability of oil should always be discussed with the fuel supplier. A number of oil suppliers do recommend additives for vaporising appliances. AGA only recommends Total Glomax which should only be used once the tank has been cleaned and flushed prior to its use.
Use the 'low' setting; however, if the gap between the two events is less than 6 hours then it is best not to choose this setting. The AGA is so well insulated that it takes many hours for its stored heat to reduce; therefore it would probably never actually reach the 'low' setting and certainly not the 'slumber'.