After starting my car up from cold, for the first minute or so of driving I have to stay out of boost because my turbo makes a metal on metal sound as if the oil isn't lubricating. After that it suddenly goes to the normal, pleasant whistle of a healthy turbo. Does this mean my turbo is dying? Or could it mean that I have to replace the oil lines to the turbo cause the oil isn't viscous enough on cold to get through those old, clogged arteries?

I'm no expert but I'd say it's a pretty fair bet it's on the way out. You could be hearing the impeller or compressor blades scraping against the housings which would mean the bearing surface and or shaft is worn.

Remove the hose coming from the turbo to the intercooler, reach into the turbo and wiggle the shaft. Anything more than a slight amount of play could mean it's time for a rebuild. If that's the case I'd also replace the oil feed line and make sure the oil drain line is clear. Good idea to have the intercooler flushed out while you're at it.
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Erwin in Memphis, '88 745t 190,000 miles, '95 855t 92,000 miles

It sounds to me that your turbo rotor is hitting the housing, not insufficient bearing lubrication. You can check this by removing the intake hose to the turbo and spinning the rotor by hand to see if it is hitting the housing. Do this while the turbo is cold naturally (duh). At the same time you can also check for lateral play in the rotor which will give you some measure of bearing wear. In either case I would suggest replacing the turbo cartridge. If the rotor has been hitting the housing there is probably some deformation of the rotor blades and perhaps some metal missing from the blades. This is a problem that should corrected at once as the metal has to go somewhere. If you are lucky they are in the bottom of the intercooler, if you are not lucky they are in the engine. If you have a Mitshubishi turbo and are fairly savvy you could replace the cartridge (and oil supply line) yourself in an afternoon.

But why would this only happen when it's cold? If I start the car after it's been driven for a while (i.e., still warm) it doesn't happen at all. Why would the impeller blades hit the housing only when it's cold?

You may have a choice of two problems. If it is the impeller then when cold it would have lost most of its oil film, giving it enough clearance to hit the sides. See my earlier post to another question same subject, you may have a leaking exhaust gasket that is on its way out. If it is the impeller it will have over 1/8" vertical wiggle and the blades edges will be ragged. If not ragged seriously consider looking at the exhaust manifold gaskets. If the impeller is ragged, you can replace the cartridge but it's worth it in longevity to get it balanced.

cheers
RK