Engine oil begins to break down when temperatures of the oil exceed 240 F and the hotter the engine oil the more the engine oil breaks down.
When the engine oil breaks down it causes oxidation, which in turn forms deposits, varnish, and increases wear on the engine bearings and parts.
So you want to keep check on your engines temperature because not only will the high engine temperature break down the oil the high engine temperature can lead to warped engine heads and engine damage.
During extreme cold the engine oil becomes thicker which makes the engine oil harder to flow through the engine so that's why it's a good idea to allow your engine to idle for a few minutes to warm the engine oil up in extreme cold weather before taking off.
If you do drive off right after starting the engine just idle around town for at least 5 or 10 minutes before accelerating the engine.
This allows the engine oil to get up to temperature so it flows more freely through the engine.