How do additives for automotive oil work?

The main job any engine oil additive performs is changing the oil's chemical composition to solve certain problems that have come up in the engine. Mind you, many additive manufacturers cynically swap the word "problems" for the word "tasks" in their advertising — see the difference?

Say your engine has started using more oil than it should over time. How do you fix this problem? There are plenty of ways, but the simplest and cheapest is to pour an additive into the oil. And what does this additive actually do? Right — it builds an extra layer of something-or-other on all the engine's internal parts, shrinking the clearances, or it makes the engine oil thicker and less prone to burning off. The effect is achieved: oil consumption really has dropped significantly. For how long, you ask?

A car's engine is a very complex mechanism, and every seemingly minor detail is accounted for during its design. And the chemical composition of the engine oil is one of the main such "minor details". The materials used to make the engine's internal parts, friction speeds, the compression ratio, operating temperatures under various conditions, and many other parameters differ between engines, and that's why different engines from different manufacturers have different requirements for the set of additives that can and must be present in the engine oil.

Hence the countless standards, approvals and classifications for engine oils. So what does a car owner actually do by pouring an extra additive into their engine oil? They simply change the oil's parameters, forcing the engine to run under conditions the carmaker never anticipated. The consequences of such actions are obvious; it's only a matter of time.