The turbos get very hot nestled in the V of the V6 engine, so what happens if you switch off when they are too hot is the oil in the turbo bearing oil supply and oil return pipes carbonises, restricting the flow and that eventually leads to failure of the turbo bearings. Make sure these pipes were replaced along with the turbo, and idle your engine for a minute or two after towing, after long ascents and after driving on the motorway before switching off.