Idle time allows the turbo temperature to stabilize at a safe level so that the oil that remains in it after shutdown won't "burn" or "coke" as some have mentioned already.
A turbo that's being run hard will often turn cherry red or even white with heat on the turbine end. The flow of oil through the bearings both protects them from damage and carries the extreme heat away from this area, acting as a coolant.
If that coolant flow is interrupted (as in engine shutdown) before the hot components have had a chance to cool to normal levels, you will dramatically shorten the life of the turbo.
We have a 10-minute cool-down cycle on the diesel generators I work on. From the time the operator pushes the "stop" button, the breaker opens automatically and the engine runs without load for 10 minutes, then stops itself. There are emergency stop sequences in case of abnormal conditions, but we try to avoid those whenever possible.
On the other side of the coin, you can estimate how hot the engine and turbo actually is before shutting down. If you've just finished a heavy-load kind of job, give it a few minutes to cool off, probably 5 or more. If it's been lightly loaded for a little while, as in driving back to the barn, a minute or less is likely all you need.
Even a naturally aspirated engine will benefit from a cool-down cycle, it allows oil and coolant temps to stabilize to a safe level before shutting down. Once the engine stops, any coolant or oil flow stops as well, no longer passing through a radiator or oil cooler.
Sean
