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A cold engine idling uses a bit more gas than a warmed engine idling.
A cold engine uses way more gas accelerating, producing way more unburned exhaust than a warm engine. The system puts more gas into the cylinders when cold to ensure combustion.
Remember the old fashioned manual choke? All it did was increase the fuel ratio till the engine was warm. The modern fuel controller does the same job.
Keep in mind, it's not just the oil and pistons that need to be warm, it is also the catalytic converter that DOES NOT work when cold. It, and the cold muffler will condense out a lot of water if pushed and that will rust out the exhaust system in no time.