A turbo spins at about 150,000rpm and when you are on a long ascent, when you are towing or when you are at constant speed on the motorway, even a watercooled turbo can get red-hot. So in these circumstances it is always wise to idle the engine for a minute or two before switching off (when stopping to re-fuel, for instance). Not necessary at the ends of most journeys because they are usually at low engine speeds through a town and that does the same job as idling. For only 6000 miles a year a petrol engine makes more sense than a diesel anyway.