Joined: Nov 2016
Use the recommended oil, when possible. The manufacturer builds these engines with tolerances set up for a particular viscosity.
The gaps engineered into your engine are designed to leak oil out at a given pace, allowing new, fresh, cool oil to replace it.
If you put something too 'runny' in, it will squirt out faster than intended, or may not deliver adequate bearing support in some areas. Your pump may not be able to keep up with demand.
If you put an oil that's too 'thick' in, it will have trouble making it down the passages, creating similar problems but for different reasons. You'll have oil starvation at higher RPMs especially.
The only time you'd normally want to mess with that would be in special circumstances - for example, track racing. You might want to step up to a 'thicker' oil that can handle 105 degree runs around a track. When your oil gets incredibly hot from all the full throttle use and blazing summer temps, it won't become too runny to cushion your engine parts, where the stock oil just might.
Another example might be if you live in Alaska, and you routinely start and drive your car while the temps are well below zero. Your factory oil might end up thick as taffy, and unable to do its job.