Mobil 1 is usually the highest recommended oil I can find and is what I run.
Synthetic oils give a more consistent viscosity over the varying temperatures. Translation: temperature variations don't cause a decrease in the oils protecting abilities or an increase in strain on the engine.
Synthetic oils tend to hold their properties longer than traditional oils. Translation: still change it at the recommended intervals. Sure is theoretically lasts longer, but why not be safe anyways.
Synthetic oils cost a lot more. Translation: buy synthetic.
Synthetic oils are "thinner" or have smaller molecules or something. Translation: they are more likely to leak. Usually it is not recommended to run synthetics in older cars because they'll just leak away at a fast rate. They can seep through smaller cracks, etc. Keep a closer eye on your oil level if you run synthetic.
There are synthetic oils, there are synthetic blends, and there are natural oils. The best protection tends to be had by the full synthetics. As far as additives go, I wouldn't be surprised if all oils had additives in them, but I do not know.
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