system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The ratings of engine oil, i.e. 5w10, 5w30, etc.
gpt
These oils with 2 numbers, like 5W-30 are called "multiweight" oils and protect the engine under a wide range of operating temperatures. 

This rating indicates the engine oil thickness when cold (the first number) and when hot, at an engine's normal  operating temperature (the second number). The higher the number, the thicker the oil. 

Thin oil flows better, but does not protect the engine as well as thick oil, under heavy loads. 

So we want an oil that is thin when cold, so it flows into all the tight spots in an engine on a cold day start up, to lubricate the engine better. 

But we also want an oil that behaves like a thicker oil when its hot, to protect the oil under heavy operating loads. 

These multi-weight oils provide such a wide range of operation. Engines are usually designed to use a specific thickness of oil. So look to the owners manual to see the type of oil suitable for your car.