API oils are not formulated to clean, that's not part of any standard. What they ARE formulated to do is keep things clean; not produce significant deposits during regular use, though that may not actually happen depending on the application and usage profile, as the additive package can get overwhelmed and once that happens, so do deposits.
Your typical API oil isn't formulated with anything polar enough to do cleaning. Group II/II+ and Group III/III+ base oils have poor solubility.
- Dispersants: These prevent the agglomeration of particles that make their way into the oil so that they don't ball together and become heavy enough to fall out of suspension.
- Detergents: These attach to contaminants as they enter the oil, keeping them in suspension and preventing them from plating out on surfaces.
These products are not designed to liberate existing deposits from surfaces, drawing them into the lubricant.
If you have some soft deposits, it's possible that clean oil flowing over that surface could pick-up some of this, but anything that's really "there" like varnish/lacquer/sludge needs to be either dissolved by a solvent, which carries with it some inherent risk, or attacked by something polar, like an ester or AN, which will actively (but slowly) pick apart these deposits and draw them into suspension.