An engine at idle has minimal load, so it generates much less heat and heats up slowly. The slow-flowing thick oil still has plenty of time to be moved past the critical hot areas without cooking off and failing, and the oil gains heat as it does, losing viscosity and speeding up as it does.
For the illustration’s sake, an idle engine might need one bhp and generate around 1.5kw of waste heat.
An engine driving a car at 70 mph could quickly be trying to dump 30 kW of waste heat.
So, after a few minutes of warm-up, if the car is now driven, although much more waste heat starts to be produced, the oil has had a chance to warm enough that it can flow fast enough to again deal with the excess heat without burning.
The engine that starts from cold and is driven straight away has that icy, thick oil, and its failure to deal with the instantly generated heat at pistons and valve guides is where the damage occurs.
Engine size is critical, though. A small motorcycle engine could be at 100 per cent output, and its smaller mass, pistons and so on blazingly hot from the get-go trying to shift the maximum waste heat it is capable of at 70 mph. In contrast, a big lazy 4. litre is now turning perhaps 10% of its total heat.
That 10% is spread over many surface areas, and even cold, thick oil can survive.
That is why little engines working harder need better oil, changed more often,
The thinner the oil is when cold, the better the engine is protected while those first few miles are covered…. This is why a 0w-40 will pretty much eliminate engine wear.
Or… a gentle warm-up with no load…
Fifty years ago, tiny engines wore out so fast because old tech oils didn’t protect them until the oil warmed and thinned out.
Modern oils are so much better, especially being thinner when not at working temperature.
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