A finished engine oil is already a balanced formulation. Its detergents, dispersants, anti-wear agents and friction modifiers have been chosen to work together and to stay within limits on ash, phosphorus and sulphur.
An aftermarket additive enters that balance. At the right treat rate it adds to what is there. At several times the treat rate it can compete with the oil's own chemistry for the metal surface, or push the oil outside the limits its specification relies on.
What we check
When we set a treat rate we look at more than the property we want to improve. We check that the treated oil stays clear in storage, that its viscosity stays in grade and that the elements which matter to exhaust after-treatment remain within the usual limits.
The practical advice is simple: measure the dose against the sump capacity, and do not combine two products that do the same job.