system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do I need to get my oil changed in my car?

What exactly happens to the old oil?
gpt
The purpose of oil is to lubricate and aid in keeping engine parts cooler.  It forms a thin barrier around things like bearings to prevent excess friction.  Over time the viscus property starts to decay and the parts basically rub against each other and start to wear out and thus the requirement to change oil.