Lugging is applying a high (relative) load at low rpm. If you have to put your foot in it to get it moving, you are putting more load on the crank bearings than if you drive with a light foot. Automatics provide some protection over and above a manual transmission vehicle (which can be in the wrong gear), but it is the same idea. Use a light throttle foot, low revs, until it is warm.

Lots of people warm up their vehicles at idle, not knowing any better. Most of them are convinced that they are being nice to their vehicle. The problem with common sense is that it isn't always common practice, which is to say that lots of people doing the wrong thing doesn't make it the right thing. Not idling a vehicle has been the recommendation of automakers for decades now, but old habits die hard.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue