Everything (almost) you read on this thread is true (lack of heat, washing down of cylinder, blah blah blah). Another down side is not only the lack of fuel being burnt in the combustion chamber, but also the amount of sulfuric acid generated in the combustion process (sulfer in fuel+water as a bi product from burnt/unburnt fuel) usualy increases during cold idle times.
As for the tranz. Sitting in park all day long will not get the fluid warm because it isnt circulating. You migh absorb some heat from the flywheel to the converter this way, but thats about the extent of that. Start the beast in "N" and let it idle a second. NEVER drop it out of O/D. Theres a big a** spring inside the transmission and it takes alot of pressure to compress it and make the O/D turn off. That extra pressure is taking away from the regular pressure the transmission needs to survive. The Cummins will take it, let it lug and go. Anything you can do to save wear and tear on the weak link is a good idea. I went as far as to re-adjust my neutral safety swith so the truck only starts in "N" now. That way I never forget to drop down to "N". It also makes for one heck of an anti theft device that my insureance company gives me a 10% discount for. Saving money, preventing wear and tear... doesnt get any better than that. Good luck.
Attention: TDR Forum Junkies 
