[blue2000s]

The friction and compression of air in a turbocharger generates emmense amounts of heat. Stagnant oil at the temperatures at which a turbos surface can reach will solidify and/or sludge. The next time the car runs, there is less oil flow due to the clogging created by the previous heat cycle. The lack of lubrication increases friction and temperature even further. This is what kills a turbo, manifolds and seals. So the reason it is recommended to let a turbocharged car idle before turning it off is to allow oil and coolant to circulate through the turbo in a state in which it is not producing heat, bringing it's temperature down to a level that makes it safe for the oil to sit in it. It is perfectly safe to turn of a turbocharged car immediately after a low rpm/load run on the highway or down the street. I've seen a turbocharger on a Toyota last over 350,000 miles in a car that was mostly highway driven, the owner drove like a granny so he didn't let it cool down, and he also always used conventional oil.

Likewise, but to a much lesser degree, a high load on a normally aspirated engine increases it's internal temperature. For the same reason, it is good to let an engine that has been under heavy load cool down before shutting it off to avoid oil sludge. But there is absolutely no reason to regularly let a moderately driven car "cool down" before shutting it off. In most cases, it's not cooling down at all.