2. On descent (for the circuit) reduce MAP by 1' per minute to gently cool the turbo
I've heard this a lot and am not convinced about it at all.
True enough I'm more familiar with car turbos and not aviation ones but the car ones are very robust and can take huge temperature changes without any trouble at all, even the older oil-cooled (and lubricated of course) bearing types.
I suspect the real cooling with turbo piston aviation engines is more the manifolding (with regard to expansion & contraction from the heat & temperature changes) and the engine itself with the usual rapid temperature changes.
I'm quite prepared to be wrong - any ground engineers have any good information on this. And apologies for the thread hijack.