system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If oil and grease both act as lubricants for metal on metal contact; why can't you use them interchangeably?
gpt
Typically, oil is used in higher speed, lower stress applications, where heat build-up is more of a concern than stress on the moving parts, such as the fast-moving engine cylinders. Oil serves as both a lubricant, and a coolant, by continuously circulating fresh cool oil from a reservoir, often with an oil cooler similar to the radiator for water/antifreeze. Oil has to be thin enough to be pumped and exchanged. Grease is typically thick and semi-permanent, only occasionally being changed or added, and is used in more slow moving, but occasionally high stress applications such as the moving parts of the steering system.