It's been asked before, the answer is that when the engine is cold the auto box takes longer to change, effectively high-revving the engine to make sure that the engine warms-up quickly in order not to damage the catalitic convertors (which do not like the partially-burnt fuel generated by a cold engine).
There are other 'tricks' the engine does to help protect the cats when it's cold, but that's another story....
i just flick on the frost button when cold starting and running initially. This keeps the revs at or below 2k for the first mile or so.
it doesn't labour the V6 engine.
4K from cold is not doing the engine any favours!
No specified mileage. Generally accepatble as 70k, some say sooner if the car is working hard i.e. towing or min-cabing. When checking level, check for burnt smells etc as a sign of oil gone bad too early. No drain plug either, it's a metter of removing a big sump and a small sump, then cleaning a metal filter and refitting with new gaskets. Also the box is ventilated into the engine compertment via a pipe that emerges behind the engine block near the DIS, this should be left open as blocking it will mess-up the box.