[Tannaton]

The advice to allow a turbocharged engine to run for a few minutes after stopping from a fast run was to protect the oil in the turbo being "cooked" and solids building up in the turbo oil galleries and bearings. Such solid deposits would eventually block the oil flow and ruin the turbo. This was much more of a risk 20 or more years ago when mineral based oils where much more common and even then it only really affected petrol engine cars where the heat energy in the exhaust gases is much higher than on diesel powered cars.

Far from being an old wives tale, it was a problem recognised by Volvo who fitted electric auxiliary water pumps to some petrol turbo models in the 80's and 90's that would continue to pump coolant around the cooling circuit of the turbo for 30 seconds after the engine was stopped or until such time as the turbo had been cooled to a pre-determined level (coolant leaving turbo < 90 deg C) if that was longer than 30 seconds.

On modern diesel engines with synthetic oil it's not an issue you need worry about. Turbo chargers are much better designed and cooled now.