1. Some oils state fully synthetic but are not fully synthetic, they just make the standard and are lazily given the title.
2. What oil are you using?
3. Mineral oils contain no additives as they are extracted from the ground. So the additives are added it, but the bond on a molecular level isnt strong, so over time they separate and disintegrate.
4. Synthetic oils are man made and are engineered to have certain additives as part of their molecular structure. So expect better protection and performance with a synthetic over mineral.
5. I change my oil every 5K Km, and something is awry if synthetic oil is making your car perform worse than a mineral.