A quick Google search has many different sources, none of which are scientific. One is all the manufacturers that warn against idling, for various reasons based

A quick Google search has many different sources, none of which are scientific. One is all the manufacturers that warn against idling, for various reasons based on the particular model, probably. Two is the increased fuel mixture in cold weather, leading to increased wear on cylinder walls from running rich until the engine reaches temp. Three is that cars are designed to operate under median load, not idle: half of how a car cools its engine is air passing over radiator plates, and that doesn't happen when the car is stopped. The engine actually has to work harder to keep the fans going in hopes of keeping temps down (in hot environments it'll just run hard constantly). And in winter it's the opposite problem, with oil viscosity and antifreeze working at potentially the end of their design spec to both keep the engine lubricated and coolant circulating. If it had load it could keep itself warmer easier, rather than depend on all these extra factors to keep from damaging the engine. And all of this for gasoline cars; diesels have a much more difficult time, especially if they're supposed to be fuel efficient. Now, obviously I'm not claiming idling your car is going to ruin your engine, cars idle all the time with no problem. It's just not at all what the car was designed to do most of the time, and it will cause undue wear in various conditions over time. At the very least an hour of idle is equivalent to driving 25 miles (according to Ford Fleet) and your gas mileage will be worse.