I wish a definitive study would be done on this. The most recent advice seems to be run it a minute & start off easy. Theory is you will waste less fuel & the engine comes up to temp faster if it's actually doing something, the theory also is that mild work will not cause harm.

Makes sense to me & I've pretty much followed that on a number of high mileage vehicles I've owned. Who knows if they'd have gone farther with a 20 minute warm up? None of them experienced engine failure, so as far as I know they may still be running.

Dave