As a former Volvo 760 turbo driver - pre water-cooled turbo - I can tell you this. My turbo needed cooling down and I did it; 30 seconds in town, 60 seconds after 120 kph cruising on the highway. It was just about the only part that never gave me a problem in over 400,00 km of driving. Theoretically, water-cooled turbos don't need it, but I've seen advice saying it's good to do. Can't hurt.