Read any article or look up synthetic motor oil, and you will find vague references to exactly what synthetic oils are. I could be wrong, but oil having as low as 26 percent synthetic oil can be called synthetic.
As far as oil flow properties go, you see 0w-30 or 5W-40 touted as an advantage because it flows better at start-up. In order to produce an oil with a bottom number less than 10, the oil has to be diluted with a solvent type fluid to achieve low flow ratings. So you are giving up lubricity when you go below 10wt. Besides the oil additive package (TBN)is what keeps the engine from tearing itself up.
0 and 5 weight oils are used to get low emissions but are not an indicator of good lubrication.