ELi5: Why do different engines require different oil?

Question

I can understand why some type of heavy machinery would probably require different oil to a lawn mower but there are lawn mowers that require different oil to other lawn mowers.
If the answer is “because that’s how they were designed” then why? Wouldn’t a common oil make more sense?

In: 2

Answer 1

A common oil would make sense from your perspective, but you’ve gotta remember that you’re dealing with engineers here.