A friend told me if the owner doesn't drive his car too often and his car is always parked outdoor exposed to hot weather, he shouldn't pump full tank petrol because the full tank will take weeks to be used up ie the petrol left in the tank will be subject to high temperature (from hot sun) for too long and its octance value will drop. As a result, if he pushes the engine hard when the octane value drops too low, it's no good for the engine, and the engine may "piang" !!!? [sweatdrop]
His advice:- if the car is not driven often, just pump enough petrol each time to last say the next 3-4 weeks.
I am not sure what he said is correct.