Back in the good old days (or were they really that good?) turbochargers were (in most cases) not water cooled. Even today some rather modern VW diesels (2.0L BKD springs to mind) have only oil cooled turbos. Not surprisingly it's these oil only cooled units that exhibit the most failures. As on shutdown there is no water to stabilize temps at or below 100C.
On large diesel fleet oil only cooled is still rather common. I know that the workshop manager at the mine I work in kicks up a huge stink if the haul truck drivers don't let the engines idle down for 5-minutes.
Given I've been driving a turbo charged petrol cars daily for the last 12 years here are my tips.
-If the exhaust housing is glowing red from copping a beating. Best let it cool down a little before you turn it off.
-If you're just highway cruising or pottering around town, don't even think about it.
-Use high quality Synthetic oils (Helix Ultra for me or similar to manufacturer specification.
-Change at 5-10,000kms intervals depending on vechicle usage. Oil is cheap, turbo's are not. even the smallest amount of grit can damage bearings in turbo's. Clean oil (and blow off valves) keep turbos from destroying themselves.
I personally maintain (from my experience) that my fuel economy begins to deteriorate beyond ~5,000kms (in my turbo Saab 95.) Sure the oil will go another 5k. but with the increased fuel burn, I'm better off changing it out as I make it back in fuel savings. And that's at Aussie Oil prices
Seen too many people wait out oil changes on turbo vehicles to save a few penny's only to have to spend real $$ replacing a turbo.
Keep Boosting
Jordan