"At absolute zero (-273 degrees Celsius, -469 degrees Fahrenheit) when all atomic and molecular activity stops.
But that is not what you are asking. Motor oil, even so-called multigrade motor oil like Mobil 1 5W-30, looses viscosity the hotter it gets. Your oil is thickest (highest viscosity) when it is coldest.
The reason to warm up the engine before "gettin' on it" is to allow thermal expansion of the block to occur. Combustion temperature will heat the pistons, and make them grow in diameter, very quickly and effectively. If the block is not at operating temperature, you can scuff or even seize some pistons, not a pleasant occurrence.
So you should really be watching the coolant temperature, and let it get above 160F (the closer to your normal operating temperature the better) before hammering the throttle.
Bearings love lots of cool, thick oil, and as long as the oil pump can pump it, the cooler the better.
(Please note, in a past life I was the Director of Research and Development for the Clevite Bearing Co., hence my screen name.)"