I know it's no fun but, read the manual for your car. (-; Subaru puts
proper care instructions in there if you really want to know.
Your electric cooling fan(s) are temperature based, but also come on if the
air conditioner compressor is in use to keep air flowing over the condenser.
It's not unusual for a cooling fan to kick on when your car stops moving so
I wouldn't recommend using your cooling fan(s) as an indicator of whether
your turbo is hot or not.
To try and help explain what is happening, your turbo uses the oil to
lubricate and cool the turbine bearings that tend to spin at extremely high
speeds. Yes, there is also water cooling in there which can help circulate
the heat away from the turbo even after the motor is off, but it's not in
the same direct contact with the hot bearings as the oil is. When a lot of
boost is produced by a turbo, it naturally ends up with more heat in the
bearings needing to be dissipated. When you turn your motor off, the oil
stops flowing. So, instead of distributing the heat into oil flowing
through the bearings, the oil sits in the bearings and pretty much gets
cooked by all that heat. The term used is "coking", which is your oil
forming deposits as it breaks down in the hot turbo. Eventually this can
lead to clogged oil passages, but even if they never clog entirely, would
you really want to decrease oil flow to your turbo?
I doubt it eliminates coking entirely, but synthetic oil tends to coke much
less. Also, consider having a turbo as severe service for your motor oil
and change it accordingly.
From all I've ever heard and read, letting the motor idle for 30 to 60
seconds before shutting it off should be sufficient to prevent coking under
most conditions. But, use your own judgement. If you were driving 30 mph
without producing boost for say a minute or more before parking, there would
be little reason to idle longer than about 10 seconds to give the turbine
long enough to spin down to it's idle speed, but if you just went up a steep
hill with full boost, sure, idle the motor before you shut it down.
All the turbo timer is going to do for you is keep the motor idling for a
specific amount of time after you take the ignition key out, somewhere
between say 30 seconds and 5 minutes, depending what you set the timer for.
If you have the money and don't like to idle the motor yourself, might be a
good investment, otherwise, is it really that big of a hassle to idle your
car for a minute before you shut it off?
Wikipedia has a more in-depth article on turbochargers, with pictures, if
you want further reading:
http://en.wikipedia.org/wiki/Turbocharger
~Brian