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Originally Posted by mtiede  What damage can be done, why and how? |
Not only is there a very small airflow over the engine and the fact that it will tend to breathe in its own exhaust fumes (Especially not good in Ynot's case as his filters are nice and celan), contaminants (Normal by-products of the combustion process) build up when the oil is cold destroying it on a normal startup its not a problem because the oil is quickly brought up to a temp where these are boiled-off so to speak but at idle and especially on a stone cold engine the oil does not heat up very quick and the crap it gets polluted with has sufficient time to start breaking it down.
If you do this warm at a standstill often the oil will break down to the point where its no longer capable of protecting the engine to its specified viscosity rating and the real damage occurs.
Much better to start the car up then if you need to scrape ice from the windshield or whatever go ahead and do that 2/3 minutes wont hurt then drive the car moderately and allow it to come up to temperature while moving if you do a lot of short trips then its a good idea to take the car on a good run every week to get rid of all the crap.