Full syth oil are made of highly refined, but nevertheless conventional carrier oil. The remaining percentage is still based on "conventional" oil, though it has been chemically separated and "synthesized" into PAOs and other synthetic lubricants. ALL MULTI WEIGHT SYNTH OILS ARE DETERGENT BASED OILS! The detergents are what clean the soot and particles from the engine, getting stuck in the oil and thus turning the oil black.
only oils that are not detergent bases are single weight oils like 2 stroke oil or sae 20w and the likes.