A few things I have observed over time. Control of the alloy used in casting may have ben poor. One member did several hardness tests some years back. The tests showed a substantial difference in Rockwell hardness numbers. Between 603 heads.

Plus most have used these engines like non turbo gas engines. Letting them run a minute or so before shutting down does tend to equalize hot spots. The coefficient of expansion and contraction is substantially greater than with cast iron. So if hot spots boil off the stresses will be substantionaly more and over time can take their toll. .

Also there is the turbo bearing and seals to consider. As soon as you shut down the cooling effect of the oil flow is lost. Some of the latest Volkswagon gas cars have added an electric oil pump that cycles on for a minute when you shut the engine down. To service the turbo and bearings by absorbing heat.

If people persist in not observing the requirements of tubo engines from an operational standpoint. Especially those with lets say inherent weakness issues. There is a price to pay in longevity. This also is a good practice with even the all cast iron engines.

It is a hard habit to develop. Plus there is no certainty. At the same time leaving the engine at idle before shutting down is not going to hurt anything. Especially after a highway run.

Driving up to a fuel pump when just pulling off the highways. Shutting down right away to refuel is perhaps about as abusive of a diesel engine as possible. Instead hang back and let the engine idle for a minute of two before pulling up to the the fuel pumps and shutting down to refuel.

For the cost of the additive a water wetter agent might also be considered. Every attempt to prolong may be a failure at the same time. As these engines do not even appear to like moderately high operational tempertures. This to me also is an indication that hot spots may be boiling off during normal operation. As was mentioned by an earlier poster.

If this is the case another contributor is. Test your rad cap for its release pressure. A low pressure cooling system just might have a little influence on those hot spots. Higher coolant pressure may reduce the extent of boiling off hot spots to some degree. Again this is something that is far from certain.

There are other reasons to not want a tired radiator cap with age. So you can get them tested easy enough. If I did not have a pressure tester I can see a garage doing this for me for free. Or a rad shop. It may be fine but untested there is no assurance it is. You just take your radiator cap in.